Depends on my reserve.

Typically I'll keep around 0.15 - 0.5 when I'm under 3.5BTC. Above that I start betting 1-2BTC at a time.

When I'm over 10-15BTC I do crazy bets of 3-6 depending on the max (I play roulette a lot).

Also depends on how my gut feels. I trust my instinct a lot when gambling since it's all luck anyway.