There are two day cruises that go mostly to Zeebrugge, or for four days you can go the likes of Amsterdam & Channel islands. A week will allow you to go to the Norwegian fyords or northern Spain, two weeks will give you a med cruise round Spain, France, Italy or Canaries or Baltic to Russia. If you want longer you can go round the world.
Depends on what you like. If you like freestyle and relaxed and not so formal NCL have 2, 3 and 4 night cruises out of southampton in May and June. If you like the WOW factor and popular choice, slightly more formal then try RCI - they have a couple of 3 or 4 night cruises out of Southampton on their Ship Independance of Seas. Or more formal try P & O. who have various 2. 3 and 4 nighters throught the year.
If you can go for longer than 7 nights - go for it. We went on a 4 day cruise with Royal Caribbean for a taster and regretted it - not because of the cruise , which was superb, but because it was far too short!!!! 7 days doesn't cost much more than 4, especially with the deals around just now, and much more worthwhile. On our 4 day cruise we departed at 5pm, had 2 full days and berthed at 7am on the 4th day, so it was really only 2 days and an evening. That said I would definitely recommend RCI - in fact we are going on a 14 day cruise with them this summer! Check out the websites and compare before booking. Whatever you choose - enjoy.