What dose the law state about how long you have to be of work to reset your perdiem clock

I don't know because I don't know where you are - for that matter, I'm not sure what you are asking.

Per diems are generally part of an employment or labor contract. Not statutory law. So you need to give us more details.

In the employment context, per diem often refers to payments made every day in addition to an hourly wage or periodic salary. Most often, it is to compensate employees for room and board when they have to work away from home. But what do you mean by per diem clock, and what to you mean by re-setting one?

I'm guessing that the OP is asking about the tax issues of per diem pay. Per diem for working away from home is non-taxable as long as the job is "temporary," which the IRS defines as up to one year. If you continue to work on the same job after that then the job is no longer considered to be temporary and per diem pay becomes taxable.