How long until I have to pay again
Quick Answer: a long, long time.
Specifically, i'm in "development" stage right now, and I'll be there for probably another 6 months, or more, it is difficult to tell (As of May 2012)
then comes alpha stage: that can be a year or more, maybe a LOT more.
then comes beta, which is probably less than 6 months
then RC, which is less than a month probably
then is 1.0 "golden master"
then AFTER that, is "the dot releases" which would be "1.0.1", "1.0.2" etc
you will get ALL that for free with the license you buy now.
The only time I will charge you again is for a "1.5" or a "2.0" release. And I even feel bad about that, but if I didn't charge, I wouldn't be able to write "2.0" in the first place, so you have to want it.
Alternately i may switch to a subscription model, since it is in constant development.