How far back does the Fenix store weekly and monthly performance data (e.g. running sessions)?

Activities sync'd with the cloud (Connect) are stored indefinitely, and they go back forever unless you manually delete anything.

But the number of activities saved on the watch (before the watch starts deleting older ones automatically) I think depends on the watch - when I transferred my activities off my VA3, I also only got the most recent 90 days of activities or so. But my Fenix 6 I think stores like 100 activities on it before it starts deleting the older ones.

In any case, it doesn't matter much as long as the watch is getting sync'd with Connect. The activities will forever be viewable on the Connect apps. There's not much value in having older activities stored on the watch for long periods of time, unless for some reason you like to go back and view really old activities on the watch screen.

Why does it matter having it on the watch if you can access the data in Garmin Connect?

Because that's how I want to access the data ....

Fab many thanks I thought that might be the case - I wasn't sure if it was dependent on time (i.e. it keeps it for a number of months, say a year) or the number of activities

I've currently got activities back to late July, when I did a factory reset (misbehaving during/after the ransomware attack). In the past I've had it go back about 11 months - although I think it was the number/size of activities rather than time that was the limiting factor