May is maybe the best time of the year to see Cinqueterre: non too hot, not so crowded as in summer, but with long and warm days. You can swim into the sea too, and is the best sea in northern Italy.
Consider 2 to 4 days to enjoy Cinqueterre at their best.
Or 1 to 2 days for a very short visit.
Or half a day for a "touch and go" (which I would't suggest you in any case).
From Rome with train you'll need from 4 to 6 hours: you can use fast train (Eurostar Freccia ROssa) until La Spezia, then reach Vernazza or Monterosso with a local train, check www.trenitalia.it for timetables and buy tickets.
The train itinerary on the tirrenic coast gives you the opportunity of a stop of couple of hours (or better an afternoon + night) in Pisa.
An example:
Roma Termini 18:00 --->La Spezia Centrale 21:05
Change train
La Spezia Centrale 21:21 ---->Vernazza 21:41
(price is euro 41 in second class with seat booking mandatory, euro 62 in first class; cheaper price (more or less -30%) if you don't use an eurostar train, but consider a longer trip)
I'll return soon with suggestion about sleeping.
[ Edit: Edited on 23-Mar-2009, at 04:44 by chiado ]