How far back is your Apple Watch activity history stored?

How are you looking for it?

I just went to the Fitness app on my phone. I tapped on "Activity" on the Summary Page. On the activity page, at the very top, next to today's date is a tiny calendar icon. If you tap on that you should be able to go back as far as you have data. I have data going back to 2015.

AFAIK, it will go as far back as it has input data sources. See the 'Check your Activity history' section in:

Use the Activity app on your Apple Watch - Apple Support