I doubt if it even closes. It will certainly be open at 7. That bahnhof is fairly busy with commuters at all hours of the day. The prices, if I recall, were not really much different from the prices on the local economy.

Now if you wanted to kill more of the 5 hours, you could hop on the train and ride the 15 minutes to the Hbf, where there is another decent sized supermarket. I don't know downtown Frankfurt that well, but there is certain to be a C&A or a Galleria Kaufhaus within short walking distance from the Hbf.