Hook the outflow to a garden hose, and test it out on a bucket. (say a 5g bucket) The pressure it sends the water through the hose at should give you an idea of what kind of head pressure you'll have in the tank (assuming you're holding it at the approx. height of discharge to the tank) and timing how rapidly it can drain the bucket will give you some numbers to work with to figure out the gph. If it drains a 5g bucket in 10 seconds, I wouldn't be using it on a 55g tank. (it would have to be pumping 1800 gph to attain that speed... and its quite likely MUCH less than that) If it takes it around a minute to drain the bucket, you'll be looking at 300-350 gph, which I feel would be safe for a 55g tank.
I do have to ask a dumb question though... why only run an auxillary filter for a few hours a day? Is it going to be mechanical filtration only? (because beyond using activated carbon and filter floss... there's not much you can achieve with a few hours a day... it certainly wouldn't support bio media) Also, does the filter utilize water cooling for the motor? Are you planning to break it down and clean it out before using it on the tank? I'd at very least run it in a bucket of HEAVILY dechlored water for an hour or so to make sure there is NO residual chlorine which will leach into the tank... but I'm not sure what else you might want to worry about.