How long is a single re-entry permit valid for if I leave Thailand?

ALL re-entry permits (single or multiple) are valid as long as the current stamp in your passport is valid for.

You can be out of thailand until the day the re-entry permit expires then you lose what ever stamp you had that you got the re-entry permit for.

Remember a re-entry permit doesn't ADD any days to your current stamp, it only lets you leave/come back and still get stamped in until that date..