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.