Data source: this Reddit poll that I made some time ago: https://www.reddit.com/r/poland/comments/x297te/do_you_tip_in_restaurants_in_poland/ .
Tool: Google Sheets.

The results must be treated with a grain of salt: I got 2393 answers and the subreddit's community is most likely far from being a representative slice of Polish society. The poll was also quite simple due to Reddit polling limitations.

Nevertheless, the results are interesting. The respondents were divided almast perfectly into thirds both on the amount they tip (10%+, just a few PLN, nothing) as well as the frequency (always, only when the service was exceptional, never).

Bear in mind that there's no tipping culture in Poland akin to USA. Tipping is optional and happens mostly in restaurants with service if ever. Looks like it's a hot and divisive topic anyway.