Actually pretty simple - MSR's own stuff says one of the 227 ml canisters will last 90 minutes on the stove. So, I'd say *IF* your estimate of 40 minutes a day is correct, you need roughly one 227 ml canister for two days, so your 8 day trip would need four canisters.

Now the kicker is - how did you arrive at that 40 minutes figure.

Now me, personally, I take trips using a similar stove (Snow peak) and I figure it out by how much water I will be boiling, and basing it on my experience with fuel consumption in the past - how? I a brand new a 227 ml canister and put it on a postal scale - record the weight. I then take it on the trail in real conditions for a couple of days. Bring it home. Record the weight - do the math. You can quickly arrive at what your *actual* fuel consumption will be and plan it pretty carefully.