Hi, long time lurker here so I'd thought I'd try to contribute
I'm a 737 Skipper and have about 7000 hours on a variety of different aircraft. I also had a flight instructor rating many years ago but that has long since expired.
I've not flown anything like a Cessna for about 10 years but basically the idea of a trial lesson is for you to have fun (and entice you into doing a PPL)
If I recall correctly it will be about 30/45/60 mins depending on what you are willing to pay. Regarding the cost, last year I got one for my girlfriend so she could get an idea of what I do. I think it was £220 for an hour in a C172, which is a four seater light aircraft. That was from Cambridge aerodrome.
During the lesson what you do will be pretty much up to you, if there is anything (within reason) you wish to see from the air you can do so, also the instructor should give you a basic explanation of the controls and let you try the fly the aeroplane. This will probably entail keeping the aircraft in level flight, climbing, descending, turns etc. Simple stuff really.
If your feeling really adventurous you could probably pay extra for an aerobatic flight but I wouldn't advise it for your first go. On a normal trial lesson you could ask your instructor to demonstrate a lazy 8, which is a very mild almost aerobatic manoeuvre.
Flying as a hobby as very expensive, commercial training more so. If you wish to get into flying for fun and don't have very deep pockets I recommend gliding. Its as much if not more fun than flying powered aircraft but significantly cheaper (Think £600 for a license as opposed to £6000 for a PPL, those figure are from memory and well out of date, a PPL could cost a lot more now)
I you have any interest in flying I say go for it, you'll have a great time.
Hope this helps.