Unless the T2 and S6.5 are drastically different in their efficiency numbers (which I doubt they are) the you should expect around 1138 shots.
There was a test on PbNation a while back that tested the T2 efficiency and they said you could get 1720 shots off a 68/4500 fully filled. So translating that to a 45/4500 you should theoretically get 1138 shots.
That is assuming your reg is sweetspotted perfectly, you aren't auto triggering too rapidly, and you could empty the tank completely (which you couldn't because the operating pressure of the gun is higher than the last few hundred psi in the tank). Also, the tank would need to have a full cold fill of truly 4500 psi, and not a hot fill where the pressure will drop after being filled.
I'd say 1000 shots off a 45/4500 with a tuned S6.5 is possible if all the above are true.
Here is the thread I mentioned earlier if you wanted to read it. CCM T2 Efficiency Test results