There are no hard and fixed rules, although there are some generally accepted standards. Actually, a lot depends on how tall you are and what kind of work you are doing.
If you are doing small detailed work you may want a "counter" at about 40 inches off the floor.
A standard woodworking workbench is at about 36 inches.
The Black & Decker Workmate is at 31 inches and can even be dropped lower. Then you can always cheat the height with a stool. A workbench that is killing your back because it is too low becomes higher when you sit down on your stool. In fact, an adjustable height stool can often give you the flexibility you need to be able to move from small detailed work close to your face to something with your arms working hard much lower and then you can stand up to get more height over the project.