i would find a local studio which isn't too ... weird ... and see what they are offering. i like having an instructor to guide me through class and others to take pointers from while in class. studios can be costly, but it motivates attendance and they will help maximize your experience through adjustments mid-class.

3-4 times is probably the best you could hope for because the body needs to rest, unless you are alternating between different types of class.

i've done yoga off and on for the last couple years and it has helped greatly with my mild back issues. there are others on here who can speak to it in much more detail *coughshellscough* and probably give you more detailed info.