It's being updated regularly - patches appear very often - sometimes with days in between, but larger update versions take more time.
If your question is how often are notable updates, I would say a few months between at least.
No, once you purchase, you've got it forever - including every update in the future. If you purchase now, you can see how often the updates are, too, so don't worry. I'd say, personally anyway, probably best to buy now, to play it, and then return to download the newer version to try any newer stuff.
In other words, if you purchase now, and it updates in a few days or a bigger update in a few months, you do not need to pay again; you just download the newer version easily.