I mean just make the options all checkboxes so people can specify multiple choices. I don't know if this system allows that for polls, though.
People might then select them all. To avoid this you could have say - 2 choices max. But then - you could ask - well of those 2, which do you like best?
So maybe, if 1 isn't enough, "other" and stating the time-frame in the thread might be the best option.
The poll & the following discussion should kind of be of equal value anyway.
The only problem is if for some reason, certain people would prefer to keep their preferred timeframe a secret, though i don't see why they would.