No set time, Miss Prim. Modern PVC cables are very durable. There's nothing to be gained from setting a time limit.
A periodic test of the whole system should be carried out about every ten years. If you're in any doubt, and you don't know when the last test took place, then it would put your mind at rest to have one now.
The test covers all your circuits. If any cable or fitting is breaking down, it will show up. That's the best way to do it. There's certainly no need to rewire just for the sake of it...... if everything tests out OK.