I can only comment on PICU and my experiences there as a resident, but there's generally been a trend towards shift like work (I assume you're asking about the attending's schedule). Obviously this setup will vary by institution and by the number of people in the group. At my place, there are seven intensivists for our 19 bed unit. They do one week of day shifts (7a-430p), then the night coverages comes in. The night shift varies, but generally they are not on for the whole week. Saturdays tend to be 24 hour shifts.
When you're not on service or covering nights you are off, though our PICU guys do cover the sedation service as well. It's actually a pretty good gig.
Some larger units employ NPs to aid in coverage, though the tendency is still to have an attending physician in house 24/7.