Cycling can go fast, or take a long time: you definately cannot bet on a standard amount of time. You just need to watch and go slow.
Honestly, I wouldn't add that many critters to the tank all at once, especially if you won't yet have a fish to clean up after. Go little by little, taking care to do full a acclimation process per each species' needs when you do add them. Also get a mix of snails: they eat different things, so lots of one kind will miss out on certain things.
Here's what I've learned on clean up crews so far (people disagree if I'm off base):
Mexican Turbos: a good idea, but don't overdo it on numbers or some will starve. Just one to start in a 55gal and see how it does. They are really fun to watch too with their big bugeyed ET heads.
Astrea: I've heard that these have problems turning over if they fall on their backs. I have just one and it has not had this problem yet, but who knows.
Margarita: Seem to be decent LR and glass cleaners. These I have had fall on their backs and can't get up... though I may have other problems causing this in my tank.
Cerith: supposedly do not have the turnover problem, and will stick to glass and LR duty, taking care of diatoms. The two I tried to add didn't make it, probably because I only acclimated them for 2 hours (rookie mistake). All snails are highly, highly sensitive to quick changes in salinity, so take extra care with them.
Narsarius snails: will dig in your substrate and eat food that lands on it. Fun and personable in the way they burst in and out of the sand, zoom around much faster than most snails, and look around with their little periscope siphons. A win win species IMHO
Conches: I've not tried one yet, but they are supposedly worthwhile. Most rarely climb, are neat looking, and will prowl your substrate.
Hermit crabs: go small. The bigger they get, the more likely they are to try and sample things that you don't want them to. Most healthy snails are safe, since they can seal up against attack, but there are no safe crabs and some people forgo them. However, the majority of folks seem to have them, and rarely have any problems. 15 is as many as you would want in a 55gal though at least to start (don't listen to the "1 per gallon! claims that some LFS will try to use to get you to buy a ton). Don't forget to make sure that they will have access to different size empty shells to transfer into when they grow, or else they will get desperate and eventually go after snails.
Emerald crabs: can attack other livestock if they get big enough. They are rumored eaters of bubble algae, though YMMV (mostly it seems to be whatever the individual crab decides he's into). Mine has yet to sample any that I've seen in a few weeks. Banish to the fuge if they ever get too adventurous.
Shrimp: I've not had any of these. Most of what I've heard is that they do more eating of extra food from you than they earn their keep in cleaning your tank. If you get fish though, some species can clean fish.
Cukes: danger danger will Robinson! Not for the beginner I think: you must know about their dangers if you consider it. Some species might be safer than others, but at least read up on them before so you know what you are getting into. They are, unfortunately, some pretty darn good substrate cleaners from what I hear.
Sand sifting stars: these are hardier than most stars, and will eat through your sand bed for you. However, what they will be eating are all of the beneficial animals that live there. And in a 55gal tank, the star will quickly eat everything there is to eat, then starve to death. If you want a star, try a brittle/serpent first rather than anything else.