Is there a time limit to return to Thailand on a retirement visa with a multiple entry permit without losing the visa?

If you leave without a re-entry permit, that visa is cancelled.

Yes you need to return before your extension of stay expires.

I doubt you're on any visa. You are most likely on an extension based on being over 50, which is not a visa, it's an extension. When you purchase a re-enter permit, the end date of the permit is the same end date of your extension. You must be back in Thailand before your extension ends to apply for your next extension, or you must start over.