Right, if this is about Warfare Officers, I'll give you a decent answer...
You'll be "at sea" for at least your first 2 jobs (about 3 - 4 years). "At sea" can mean different things: week running (sail monday, back friday), a couple of weeks away, or a deployment. Whilst at sea you will be 1 in 3 on the bridge (4 hours on, 8 huors off) all the time. Around that you will have to fit in the "paperwork" side of your life. As a running average, I would expect you to get about 7 hours sleep in 24 (although not necessarily in one block), plus an hours phys a day (if you want). Alongside in a "at sea" job will generally see you working 0800 - 1600, although you will work to requirement - there is not a lot of tolerance for missing deadlines. You will also be part of a roster as Officer of the Day (OOD). Once qualified you can expect to be anything from 1 in 2 (Navs and me on a Small Ship for about 2 months) to 1 in 14 (at the end of a 8 month deployment). It depends on how (and who) organises your roster, but you'll have little voice in the regularity, although you'll probably be able to swap days etc.
Deployments happen as they happen. Everyone in the RN should spend no more than 660 days in a rolling 3 year period away from their base port; this does not apply to Officers in their first 10 years in assignment. In 5 years I've racked up 1189 days away from my base port, and I'm not particularly overworked. When I returned from my last deployment I'd accrued 42 days leave, again, not noteworthy. I've just left a ship that will be doing at least one 6 month deployment every year for 3 years, however the ship that's currently working the most away from home has been based solely in the UK for the last 14 months. Don't expect this to get any less pressurised post-SDSR: as it stands we've shed no tasks but lost 4 units, with another 4 to go soon.
As you become more senior (i.e. mid-seniority Lt onwards) shore appointments may be available, but the general feeling is that the only place for junior Warfare Officers is at sea. I would suggest that we'd probably keep more junior Officers if we didn't keep them licking windows for years at a time, but I'm not 2SL (yet), so be prepared to fight your corner. Work time on a shore draft can vary from 0800 - 1600 Mon - Thurs, 0800 - 1200 Fri (fairly rare, ultimately boring) to jobs like Flag Lt to a Senior Officer that can see you working harder than you ever would at sea.
Shore appointments for Warfare Officers typically become the norm post PWO/Ops appointment; you could be looking at 12 - 15 years in service before achieving this. Obviously, once you become a senior Lt Cdr, there are possibly only 1 or 2 jobs left at sea for you, although increasingly more senior Officers are being sent to Bahrain and Afg to fill Joint jobs for 6 - 12 months at a time.
