I only read completely scanlated series, except if it's something that's been so intensely hyped that I can't contain my excitement (which is rare) or, more commonly, if it's a strictly episodic series (by which I mean a series where there's no/almost no continuity between one chapter and the next). In which case my memory lasts 6 months, tops.

Well I can still remeber series which were not updated for a year, but the ones that had last chpater translated 450-500 days ago or earlier I can't really remember already. So I suppose over a year.

Believe me or not but I never forget utterly applies. Memory may fade after a few more decades but my brain has always been my main (or only...) selling point.
That´s one of the reasons i update databases, on the web or @ work, as i actually do remember. The same goes for TV shows and so on.
How many realistically went back for past seasons when Arrested Development (a 1 decade jump) returned and how many will for Twin Peaks (2,5 decades). I would´t need to go back just to remind myself if the unfinished Exo Squad / Gargoyles (the cancelled sequel comics were good btw.) or American Jesus / Miracleman came back soon. Thus i go back to reexperience the greatness and not to remember in 99% of all cases.
I obviously have a better recollection of 9-10/10s and not the 7/10s (usually where i draw the line for my reading / watch lists) but that´s a fault in the work and not me.

I for example read Parasyte for the 2nd time just before the anime came out, to properly judge the adaptation process, and i remembered even the most minute details.
I further recently re-played CoD: WaW / BO 1 and i could quote memorable lines seconds before they occurred and knew most levels by heart despite only finishing them once, in 2 session playthroughs, during the month of release. I even remember where i was when i read DKR, Watchmen or Berserk (weird as i ain´t crazy about Vol.1/2) for the first time. THE sign of quality writing right there! There are sadly enough train-wrecks i am equally incapable of deleting from my mind. Any gift can easily turn into a curse, so don´t envy me too much.

Gaps in publications are nothing new or problematic with fiction. Still halted high-profile works as A Dance with Dragons in 2011 or All Star Batman and Robin in 2008 come to mind and just think about the gaps between the Star Wars Trilogies or, to a lesser degree, Nolan´s Batman. European comics often have at least a yearly gap between 60 page albums and 2+ years jumps are nothing special or unexpected either. The ongoing Blacksad, Largo Winch or Le Scorpion are good examples. How could i live with and welcome the "gaps" between Berserk chapters if i though otherwise and let´s not start with "resting" scanlations.

If the story really stuck to me in some ways, I'll remember the basic plot points and maybe have a memory of some favorite scenes. Otherwise, it can be days before I completely forget a series. Sometimes I wonder if I'm developing early on-set Alzheimer's....

It depends on how complicated the story can get.
If it's an over-convoluted shounen, it can take only a couple of chapters of me not reading to forget what was going on.
If it were a nice slice-of-life though, I could jump into the new chapter with no issue.
Besides that though, with a majority of series, if there's a delay of over a couple of months, I'll have to re-read to remember what was going on, unless the story is very memorable.

I'm used to reading traditional novels, so a year between releases is nothing. Unless the manga is really unmemorable, I'll pick it up after a 2 year hiatus without much trouble.

For me it depends on how memorable a series is to me, or how invested I am in it.
With say Vagabond, Black Haze, or Baraou no Souretsu, I'm really invested in the stories so even if a couple years pass, I'm sure I'll remember what was happening in them. On the other hand, there are some manga that I'll basically forget all about after not reading them for a month.

Depends on how memorable the series was as well as on the complexity. But I forget a lot of stuff if it's been months since the last release.

couple of months generally but sometimes I just jump in and let the memories of the last chapter rise to the surface as I read on with the first few pages. If nothing clicks, I go back some. Depends on how interesting the manga is to me of course. If it's something I'm quite eager about updating, I end up itching to reread it before a new release so I don't even have time to forget.

I have a horrible memory, so I usually forget right away. When a new chapter is released, I usually reread half of the previous chapter. This is part of the reason I prefer reading completed series.

For me, it's more of how much I actually liked the plot to begin with. If I really have a thing for it, I've keep thinking about it and I'd remember. But then there are those other stories that are enjoyable but not... memorable? XD So typically, on average around half a year? (In contrast with readings I have for my classes... I read, then an hour later I don't even remember if I had read it or not...)

Well, I never forget completely. At least that situation haven't arrived just.
But, I do forget bits of a series Over Years(s). There's still the variables; like how long(chapter count) the series is or it's release frequency or how much I'm into it.

Hmm...a lot of happy ending lover, I see.
Guess a lot people of leading a pleasant life. That's nice to know.

I even forget that i am following that series if it's not updated regularly because there are many manga i read. Only series that i really like that i remember. That's why i prefer completely scanlated series or series that already have hundred chapters ahead before begin reading it.

just a few weeks typically... i have a bad memory and i read lots of the same genre so they're all kind of similar lol

This poll is quite ambiguous I should say. We simply can't count the time of how long it takes us to forget something, if we forget then we just forget. Also another thing is that there might have been some series that take long hiatus/translation time. In such case, the period that take us to forget where may depends on the hiatus period length. This could be simplified to whether one may forget a series they read after some time or not.

Anyway, this poll is quite interesting. I think the answer is very dependent on how much series one take at a time. The more series you read, the higher chance for you to forget, which means it should take you less time to forget where the series was left off. I personally think that I haven't forgotten the last state of series that I read, but I think I may if it's off for a couple of months, especially for series that I take just as a filler.

It takes me over a year to forget stories, longer if I really liked them. I read a lot of ongoing novel series when I was younger, so my memory's been trained well.