You work 4 12-hour shifts or 3 16-hour shifts a week. The 16 hour shifts are snapped up by tenured medics so you will probably not get one of those for a few years. You'll work around 48 hours a week or so... guaranteed at least 8 overtime hours per week, probably more...
Shift start times vary and are staggered throughout the day. So 0430-1630, with shifts going out every 15 minutes or so until about 10am, and then another round of shifts going out in the afternoon around 1500 all the way until 2100. Some shifts are fixed days (Monday-Thursday, Wed-Sat, etc...) and don't change, other shifts have alternating days, with every other weekend off.
Medstar did away with the 24-hour shifts about 2 years ago or so.