I did this about a year back with my UGF in my 30gal tank, it wasn't too bad. I left the fish in the tank, I thought it would be way too stressful to chase them around with a net, put them in a crammed bucket, then repeat the net hunt when moving them back to the tank.
Take a gravel cleaner sucker hose thing (I know, nice scientific name there) and give the gravel a good cleaning to lessen the load of junk you're going to get coming up with your UGF. Once done with that, just slowly sift all the gravel off to one side (if you've got more then one plate in your tank take a break inbetween them to let the water clear up), if your water starts to get really really bad, halt your mission and let your other filter get some of the stuff cleaned out and settled back down, and do a bit of a water change, then proceed on. Once you've got the gravel shoved over to one side, pull it out slowly so you don't give your fish a heart attack.
Try and suck up some of the water, mine did get pretty nasty with everything all stirred up, but it should clear out pretty quick.
Oh, and to as how long it took..it was a year ago so I'm not certain, but I think it was several hours. With a 55gal it might tank longer, but you aren't doing something the entire time, since I waited for awhile between each plate. (I cleaned house while I let water settle
)
Hope that helps.