When do you buy a game?

Depends on game and how many hours I think I can squeeze out of it.
For me AAA games are generally bought upon/closely after release date when reviews are out, while indie games are bought on huge discounts.

For me, it depends much on the game. I am more likely to buy on sale, but not always. So for example, I had been interested in Don't starve together already during beta, but didn't buy it then. But when it came out of beta and was in sale, I bought it at the end of April. But there were many I put on my wishlist in order to buy them on sale. And when sale started, I checked which of them had decent discounts. So for example I bought Darkest Dungeon, but I didn't buy Stardew Valley which only had a sorrow discount. I regretted not to have bought Lego Worlds when it came with an extra gift shortly before sale (didn't realize that it wouldn't come with an extra gift always), and so I didn't buy it at all.
One game I'm thinking to get when it comes out is Civilization VI. But we'll see - it's quite expensive, so I might wait, maybe not even for the first sale afterwards, might be even longer.

GOTY or pass, there is nothing that irritates me more then having incomplete games in my library.

Like others here I go by the price. If it looks worth the full release price then I'll gladly buy it then, but with my huge library I'm fine waiting for sales on most games.

Since I'm not interested in all these AAA titles and my genre (adventure games) is not that pricey, I usually buy the games I want on release (except for Telltale and some other episodic games, there's usually a considerable discount after a release of a final episode, and I don't play incomplete games anyway; still, sometimes it makes sense to support indie developers midway, so there are actually no rules here). As for extremely rear cases of buying games of other genres, I would never pay full price for them.

Whenever it seems worth it. Usually around 10,-
If I can get 1,- per hour of fun, it's fine. So a 20,- game should give at least 20 hours gameplay.

However if some friends buy a good multiplayer game on release, i'll smack down 40,- as well.
(Overwatch, Dying Light, GTA V)

Either when it's on sale, in a bundle or when I really want to support the developers (and want to play asap).

It depends Multiplayer games usually when they come out and single players usually when they are like 15-20$
indie games and older games usually only when they are on sale for like under 5$

Probably 90% or so of my games I get heavily on sale or bundled.

10% of the games I decide it is worth getting now to either support the developer or because I really freaking want the game.

I only activate the games I think I shall play one day and most of the I get from bundles, about 20% I bought on sales and very few ones I bought on full price, early access or preordered.

1, 2 and 3. Depends on game. The Crew or Rainbow Six were bought fullprice for example, but Grow Home in Humble Bundle and Assassin's Creed in Sale.

Depends on the game. Some games I'm willing to spend full price or close to full price on (Total War: Warhammer being a recent title I got at launch). But generally I don't pay more then € 20 - € 25 on a game.