I've been doing this for a few months now and still am at odds on when to refill. There's really no set timing on it, and that makes it a little hard for me to know when. I do know that if you are using a passthrough on your puter it'll take a lot more refilling because you go through a lot more juice with the passthrough than with batteries. Also, some of the carts may be fuller than others with juice, some people vape more often and pull harder, too. So this is a real conundrum for me.
I try to go on the change of taste, it does taste a bit marshmallowy to me, too, when they get low. Vapor production doesn't go down, though, that's what makes it hard for me.
Better to refill sooner than later with the cartos.