I just try to do a new incentive for every update! If you were to post new incentives more often than you update, you'd need people to remember to come back and vote on other days, so there's probably a limit to how much you'd get out of doing that.
You don't have to refresh them that often if you don't want to or it's not feasible, but that's like any kind of update, right? the more often you can, the better, but you want to keep it doable. Even just a new vote incentive for every first update of the month can give you a nice start-of-the-month boost!
Of course my vote incentives often look like this; I don't spend a whole lot of time on them (usually), they're just fun extra scribbles where you get to see the characters interact a little more. And I know some people who don't wanna draw extra stuff will use a tiny preview of the next upcoming page, like a single panel without the text (I've seen those do really well, people always want sneak peeks) or close ups or process shots of the current page.