At what mileage,should most cars have a cam belt changed.?

Timing Belt?

Check manual or go on line.

Default is 60,000 miles.Or, Immediately if you just bought a used car.

Some can go 100K.

Some chains last forever, some Don't.

60k?

Some are specified at 90k.

Look in your owner's manual for that. It varies 60,000-90,000 miles

READ the owner's manual that came with the car. There will be a whole chapter on maintenance and a written schedule for when maintenance should be done.

Some manufactures recommend changing it at as little as 60,000 miles and some others as late as 110,000 miles. It is critical that it be changed at the recommended mileage or at least every 10 years. In some engines the water pump is driven by the cam belt. If that is the case it is smart to replace the water pump at the same time you replace the belt. Some engine will have extreme damage if the belt breaks while the engine is running. Consult the owners manual or call the dealer to know when to change yours. 

90,000 miles. I'd make sure all the timing belt components such as the idler and tensioner pulley and the hydraulic tensioner also get changed.

Replace the water pump while changing the timing belt.

Check your owners manual to see if your car has a cam belt. If the engine doesn't have an overhead cam, it probably has a timing chain which usually lasts as long as the engine.

The owner's manual will have the information on that.  60k to 90k miles or 7 years is the norm.

Check owners manual for your particular car, but most of mine were 80k miles or so.