This is probably gonna get closed...
But you donate what you feel like donating to those who you feel really helped you out. Of course it depends on your finances, but its a personal decision. There's people on here who have donated hundreds of dollars, and a few who have even bought phones for devs, and there are many who haven't been able to donate anything at all. These devs aren't in it for the money, but they definitely appreciate some thanks for their time. Many put that money right back into getting new hardware to develop on. But if you can't afford much at all that's just fine, none of them are going to hate you for it.
But most people donate when a dev comes out with a really useful tool, patch, kernel, etc, or when their favorite ROM dev cooks up something new. The amount and frequency is of course completely up to you, there is not and never will be a guide.
We have a ton of devs already for this device, look in the development section, or check out xda or rootzwiki, other great sites. When you find yourself using certain tools, kernels, roms, themes, etc that you like, consider a donation to the devs for their time. And if you can't afford it, that's fine, just help them spread the word, and consider helping them test their work if you've got the time.
But again, no one can tell you how much to donate, its based on whatever you can afford and feel like is right.