That's what I do, PWYW on itch and paid on Steam. A time limit sounds reasonable to me, although it hasn't reallly been an issue for me, either, it's just annoying when, for example, someone on Google Play demands a $.99 refund after playing the game for two years (one reason I'm off Google Play).
For more reference, Apple has a vague refund policy and Google has a complicated one (48hr refund window, but then they say contact the developer for a refund after that, which is problematic considering Google already took their cut).
https://support.apple.com/en-us/HT204084
https://support.google.com/googleplay/answer/2479637#policy&zippy=%2Capps-games-...