about How fast in theory should you find a block with a 5850 card? ( says 1.5mh/s speed in this miner) and with current difficulty (40) ...
I have my GPU running at 3.2 mh/s and I have been getting around one block per hour. However the difficulty has been going up and I haven't had a block for the last 1.5 hour. I don't know exactly how it scales to 1.5 mh/s, but I'd guess at least a few hours.
I have the same model and I haven't had much luck getting extra kh/s out of it. At the moment it is running at 558 kh/s, so it might very well be you (and me too) have found the maximum.