Welcome aboard!
The campaigns are very open-ended - yes, you get orders to proceed to a certain area and stay on patrol there for 24 hours, but you are not required to do this. However if you are playing stock you will get some renown for doing it, so why not?
What you do when you go out on patrol is really up to you, a lot depends on what style of play you enjoy and how much you want to try to stick to some real or presumed standard of "historical realism."
Some folks play with an "orders is orders" attitude, they go where they've been ordered to go and do their 24 hours and then continue to patrol in that sector until they must return to base to re-arm and/or refuel the boat.
Some folks go where they think the hunting will be good, period.
Some folks do a little of both - for instance I will proceed immediately to my patrol grid and stay on station for 24 hours, but after that I will begin to extend my patrol to adjacent grids at my own discretion.
Regarding types of enemy ships, historically speaking the prime directive of the German u-boat campaign in WWII was to attack and destroy the merchant shipping on which Britain was largely dependent and thus force her out of the war entirely.
Unless you go looking for the Royal Navy, or are assigned a patrol grid in a high military traffic area, you will probably encounter mostly merchant shipping early in the war. Great Britain simply did not have enough ships to provide full-time military escorts for merchant shipping, even after the convoy system was fully implemented. Also since there was concern about a possible attempt by Germany to invade the British Isles, many military resources were kept much closer to home than they might otherwise have been until the threat of such a seaborne invasion had passed.
However as things progress and the Allies have more and more ships to spare for convoy escort duty (not to mention dedicated ASW hunter-killer groups) you will no doubt see more and more enemy military vessels over a larger operational area.
In fact if you live long enough, you won't even have to look for them as they will surely find you!
I don't know about campaign lengths in stock, but I assume they last until you are KIA or decide to "retire." I don't think the stock game will retire you just because you reach a certain renown level or anything, but I'm not sure.
As others have already mentioned there are tons of mods available, best to see the threads about them individually in the Mods forum. However I will second the recommendations already made on specific mods and add that if you are going to do any modding, get JSGME to make it as easy and painless as possible!
