I have a client that would like to submit for a PTE to buy some land out of the plan. If the DOL approves the PTE, does anyone know how long an exemption is good for?

It depends on what you ask for. An exemption of a practice/principle or an exemption for a 1 time transaction. A 1 time transaction is expected to be done within a reasonable time. What is a reasonable time? That depends.

Logically in a 1 time transaction if you wait too long the facts, circumstances, law and names etc might change, thereby possibly voiding the PTE. So if it is not proposed to do the transaction as soon as the PTE is issued, Why ask for it as yet?

For what it's worth I've filed 2 similar requests (to buy land out of the plan) on behalf of clients in the last year and both were rejected quite adamantly by the DOL (whose ruling also covers the IRS I believe). One client even had a very long and passionate talk with the DOL reviewer to no avail. I had told my clients that rejection was likely but they wanted to proceed anyway. In one case it took 3 months and the other more recent request took only 1.5 months. In my opinion you don't have much of a chance even if the reasons seem noble and/or harmless to anyone (both arguments of my clients that were rejected). Still I realize some clients may still want to try.