We've played that the character spends the time meditating and is immediately refreshed. So maybe 5 minutes. This is in RMSS but I think the rule for the skill is similar. We haven't played with any difficulty modifiers (though it's not something that gets used in combat or anything crazy like that) or penalties for consecutive days. My character ends up resting for an average of 3-4 hours a night, but sometimes it's 5 minutes and sometimes it's 16 hours (in which case the other players wake him up early and he's groggy all day), depending on how the dice feel. A net benefit but a little unpredictable.