It seems to depend on which Department you are dealing with. I have SC clearance from one Department yet another wanted to clear me separately.
With the department I'm registered with as a contractor, it used to be two years - now the Pass is reissued every year. If you leave a Department with whom you have been employed you would have to give back the Pass. Keep a record of the number - a new employer can re-use the info.
In some it's certainly up to 7 years (for a long term employee) with possibly mini checks on the way. But it will vary. Maybe not as long for a new/newly cleared employee.
It lapses when you leave the job/department it was granted for and needs to be reapplied for in a new job. As long as not too long has elapsed that can be relatively painless.