I tend to refill when I notice my vape is getting a bit restricted (although 1 or 2 mm would be fine also) but before it gets burnt.
You can keep the liquid if you'd like, although I don't think I'd put it back in the bottle necessarily, in case it were heading toward burnt or something, although I guess if the juice is fine, you could put it back.
I take the easy road a few different setups, but also, similar flavor profiles so if I change with the old juice in it it's not a big deal and can even taste nice.
However, as long as you aren't letting the juice get so low you are getting burnt hits, well, it's fine. Although different folks have different preferences as far as refilling. I usually will do it at 1/3 to 1/4 remaining-- IF I notice.
Anna