How often you donate to open-source, and why ?

Never, mostly because there are better uses for my marginal giving (see https://www.givewell.org).

But I do submit a bunch of PRs.

I've only really done it once, and it wasn't a donation exactly. I was more than happy to pay for the Anki iOS app after coming from the free Android version. Anki is the reason I passed university. It's the reason I secured the job I have now. It's the reason I've been able to learn a new language. I would have have paid 10x the amount for it. So while not a donation exactly (I received a good in exchange) I have been happy to pay for OSS

Cash: I donate to OSS projects at Xmas...and I do so with some news sources too. I give to projects that demonstrate good behaviours in the year - they are progressive, they are doing something I need and they have a plan on how to use the money. Some projects pass funds over to another project...if they do that then I don't donate to them and I consider the other project. I evaluate the above every Xmas...and sometimes I have to pull the plug esp. with news agencies that show signs of toxic behaviour. e.g. I stopped giving to the Guardian because their technology editors failed to write balanced articles, which then causes misinformation in the comments section requiring moderation etc. Openclipart.org also won't be receiving anything this year because they have done next to nothing at providing a solution to their "DDOS attack" and haven't been receptive to support.

Code: I make PRs, raise issues and open source my code where possible.

I give what I can, when I can (and when I remember!) to OpenBSD and Wikipedia (not directly OS, but close enough), but that usually ends up about every couple years.

OpenBSD because I use it primarily, and its always felt like they get the shit end of the giving stick despite their contributions and commercial use

I've given many open source projects donations if I'm using the software. One thing about me though, if I read that the money is going to some social engineering project, like outreach programs, you will never get another penny from me.

I give $20 a year to Mozilla for Firefox. I use it every day both on phone and on the computer. I the vision and the fact that is it not controlled by a mega corporation.