Dunno - I never used the 510 cartridges. I use Boge 3 ohm cartomizers on 510 (slim ecigs should only use standard resistance stuff which is about 2.5 to 3 ohms). The cartomizer holds 1ml and I keep it damp. I think I use 2-2.5ml per day and a Boge cartomizer works well for 10+ days for me with thin clear liquids.
When I first started I used teensy cartridges (about 1/2 the capacity of 510's) and only use 1.5ml per day.
I don't think you are using too much. If you get any over-nicced symptoms like headache, rapid or irregular heartbeat, cold fingers, hazy or ill feeling, then cut back or use lower nic.