As mentioned by someone else, 1 hour of ground and 1 hour of flight is the minimum. I usually block out 3 hours for a BFR. If you are really weak on the ground stuff, we'll spend the entire 3 hours going over that and schedule the flight for another time.
On the ground, I review Part 61 (licensing) and Part 91 (flight rules). I go over a Sectional chart and make sure you remember what stuff means, what the different airspace is, and the rules for operating in it.
I do a review of the pilot operating handbook with special emphasis on Limitations and Emergency Operations. I'll have you do a weight & balance, and either a HIGE or HOGE performance problem. We'll do a quick review of some of the aerodynamics covered in the Rotorcraft Handbook.
Then a preflight with you explaining what you're looking at, and then a flight covering most of the same things you did on your checkride - normal TO & approach, running takeoff, running landing, max performance takeoff, steep approach, quick stops, slopes, hover autos, autos/forced landings, quick stops, recovery from settling with power. Any aircraft specifics such as hydraulics off approaches, TR failures, various systems failures.
It sounds like a lot, but if you are actually current it goes pretty quick. If you have trouble we'll spend a little time trying to get the weak points strong enough for me to sign you off, if not I'll recommend further training.
Most people are weak on the emergencies if they haven't been taking any dual since they got their license. Flight instructors recognize this and expect that most BFRs are not just a test, but a tune up, i.e. we're prepared to give you some training to get you back to where you should be. It helps if your attitude is one of wanting to be safe even if it requires a little re-training. Thats the purpose of the BFR.
I only have a couple pet-peeves. One is the person who comes in un-prepared and just expects to be signed off regardless. For goodness sakes, break out the books a couple times before you show up for the BFR!
The other pet-peeve is someone who really can't fly one of the maneuvers correctly, but tries to talk me out of requiring it. Slopes seem to give some people problems, but telling me you only land on flat landing areas is not going to convince me to sign you off! If you are weak on a maneuver expect to spend some time getting it up to snuff, rather than trying to convince me you don't need to know it!
99% of the time people have no problems with a BFR so be prepared for it like a test, but also expect to have a fun flight - it's a good chance to show someone what you know and get some good feedback on areas you can improve on.