I am sure it comes down to accounting nightmares and if they let the accounts sit forever. I am sure they have a lot of people sign-up participate for a week or two and lose interest and never return. I think 6 months a reasonable amount of time.
It is not to hard to cash out ever 3 months doing the bare minimum but it needs to be done daily, it doesn't take that long, unless you try for more. daily e-mail .02, ptc .15, required .05 for ptc (for me video offers fill that) so that is .22/day then if you get your 30 posts/month in the forum that is $3. 90 days x .22 = 19.8 + (3 months x $9)= $28.8. If you set a daily (or weekly goal) for yourself you will be surprised it is easier than it seems