how much do you donate to projects?

$5 or $10 once per project, or once a year if I remember. I can't afford to have subscriptions. I only try to fix bugs if I can.

If/when I have money to spare, I share that money by a function of how much I use a piece of software, and how much the project needs funding (so a combination of costs vs income). So generally the projects I donate most to are the ones I use daily, and also are either more obscure or more expensive to maintain.

Just the occasional bug report. I don't have more to spare. Most I have ever done was to give code for a semi-big new feature back. I had developed it for work and was glad that I got permission to publish the code. Especially when someone fixed a bug in it.
I also contributed to PHP. That's the biggest project with my contribution. I also tried to make something for OpenSUSE's zypper but thanks to Long COVID lack the energy and brain power.

So and so. The projects I like a lot and heavily use I give 5€ a month, but there are very few. That would also be the usual member fee for instance for the Wikimedia foundation. Beyond that, it’s usually between a coffee and 20€, depending on how much potential I see, how much it has helped me, and how much the project needs the money. Maybe needless to say: Bug reports help a lot so I place them where I find one. Finally, when it comes to code contributions I find it hard. Usually that requires a lot of preparation and time to get into the depth of the project beyond the API/fronted. And that is, more often than not, a road block for me.
I think though, that the amount given is not a good measure on its own. It depends a lot on what one has and is able to give. There are so many people out there who earn way more than they need (including me, although I am nowhere rich). They could easily give a lot more and cover for a general user base. And the latter, making the project more popular and maybe contributing some bug reports is just as essential.