It's going to be hard to tell, as tank sizes vary slightly, and the density of rocks and such vary.
Best bet would be to measure the water you put in as you fill it, but I guess that's not an option now.
Assuming the tank actually holds 55 gallons (I think the typical 55g actually holds a bit less, but not certain - if you want to find out, measure the inside in inches - a gallon is 231 cubic inches), and that your rocks/substrate are silicate with a specific gravity about 3ish...
maybe somewhere around 50 gallons?
Are you just curious, or are you trying to figure out dosing, or something else? Most of the type of dosing we do doesn't have to be that precise.