I have a new-ish series 2 watch, I've never gotten it down to below 60%, even when I track a two hour bike ride. I just charge it for a few minutes before bed and in the morning and that's usually enough to get it back up to 100%.

If you don't use it very much, I think a series 2 watch could easily last 2 or 3 days.

They charge really quickly. I pop mine (2nd Gen) on for a bit in the morning while I'm getting ready, or in the evening while I'm reading in bed. Apart from that I wear it all day and all night, largely for the haptic notifications and alarms.