

Whenever you like, honestly. It’s mostly a nice acknowledgment to the poster that you appreciated their post. Unlike commercial social media it’s not sending out anything to your followers that you interacted with it (at least last I checked).
I think many people boost more than favorite because it functions a little similarly in regards to acknowledgment, with the bonus that it helps share the post to others which is even more relevant in federated networks than on centralized platforms.
Source is instance sidebar, but if you’re using an app that’s gonna be found in a variety of places. In Voyager for example it’s under Communities>3 dot menu in the upper right>Instance sidebar.