I suppose its hard to say, there are many factors involved. I'd much rather have been wearing a lifebelt than not wearing one, put it that way.
The bodies without lifebelts probably sank after death, or sadly before it. After suffering from exhaustion and cold the body of the person may have just gone under the surface of the water and not come back up.
Then there was the great wave which swamped the deck as the bridge sank.
Colonel Gracie claimed his friend Clinch Smith vanished beneath the water and didn't come back up, suggesting he may have died by drowning from suction in that are of the ship. It would be interesting to know if he had a lifebelt on or not, and just because Gracie didn't see him come back up doesn't mean he didn't, I'm sure Gracie was probably too busy trying to save himself. So perhaps even those wearing lifebelt's could have drowned. In the same area however
Lightoller managed too swim to the upturned boat, was he wearing a lifebelt does anyone knows?
I would say that in the end it really depended where you were as Titanic foundered. If you survived the ship sinking, and any suction that may have been involved (very little apparently as the stern sank) then regardless of wether you were wearing a lifebelt or not once you were in the water you had just about the same time as everyone else to survive, which tragically wassn't very long. half an hour at the most, and even that I think is if you were lucky. As for how long they lasted, I guess that was different for each person varying on strength, shape, age, clothing etc. A person without a lifebelt may have lasted longer than someone with one.