So far my 30mL bottles of Halo's Turkish Tobacco have lasted me around 9-11 days. However that was when I was using my Halo Triton's, I just started using a VV/VW device with a bottom coil tank (info in my signature). Luckily I opened a brand new 30mL bottle when I started using the SVD/PT2, so as soon as the bottle is gone I will know how long it will last me when I am using my SVD (considering it's the only juice I use at the moment).
It's pretty easy to figure it out (if you use close to the same amount each day), just mark how much you put in the tank each refill for the day, then divide 30 by whatever amount you use per day. You do have to make sure and take into account how much was in the tank at each filling, so the easiest way is to always refill at a specific level. For instance with my Triton's I always refilled when the tank got to 0.8ml, and I always added 1.5mL, and I was refilling twice a day so 30mL÷3mL=10 days.
EDIT - Oh and just in case you order your e-juice online like I do, always make sure and order your next bottle well before you are out