I believe it depends on how long the artist usually takes to do said thing. Like, if they have a reputation for taking a year, expect a year and ect. Or if you know they have a big list of things to do, might take them longer than usual. When in doubt, I'd just suggest noting them. I wouldn't see anything wrong with asking how its going or a wip if they've started if its a comm. If its an art trade, i'm honestly not that sure haven't had that problem before.