How long is a reentry permit valid in Thailand?

Most Visas are single entry so as you enter Thailand it will be stamped USED as it's done. Re-entry permits (single and multi-entry) can be purchased from your local Immigration Office in advance or when exiting at the major airports.

@Kathy *******

Some Visa have reentry permit included, e.g. the famous multiple entry tourist visa.

There is nothing complicated.

It is the same in your country. Or do you think I can just go there and leave and go back like a normal citizen?

If your visa does not contain "multiple reentry" then you have to buy an reentry permit. Or a new visa.

Up to you.

@Ryder ********

it's good until the date your existing current visa stay or extension allows. If you got a re-entry permit for a visa or extension stamp you already have that allowed a stay for almost one year left, then the re-entry permit is valid to be used until your original allowed to stay date, either to re-enter one time or multiple times depending on whether you get a single or multiple re-entry permit. The re-entry allows you to stay again until the date your original visa stamp or extension stamp says you were allowed to stay. For example, you have a visa you used with a permitted stay date until 15 August. You get a re-entry permit and leave the country today, and come back on 10 July. You will only be stamped in to stay again until 15 August, because that's the date your original visa allows you to stay till. Re-entry permits basically allow you to resume your original stay period as if you hadn't left.