I refill my protank once a day, twice if I'm going nuts, but I've yet to go thru two tanks in a day. My juice is 15mg.
As long as you're not having any signs of nic overdose, I don't think there's such a thing as "vaping too much". However, if you're finding you're going through several tanks a day, you might want to at least contemplate increasing your nic level so you're getting the best $ value out of your juice.