What is the required spacing on conduit clamps for 3/4" pvc rigid conduit?

Depends on install. I believe it is every 3 ft. but if it is running through joists, they can be counted. You also need them in vertical installs from a conduit box, running up a wall.

Does not depend on installation, any PVC conduit, sizes 1/2 through to 1", the maximum spacing of support shall be no more than three feet. 1-1/4" through 2" is five feet, and so on.

If this is a exterior horizontal run exposed to sunlight, get back with the length and the location, as this may need an expansion coupling, and takes a bit to calculate.