Does a Multiple Entry Permit in Thailand Have a Time Limit?

No, you do not need this report. Remember that if you return you need to update you address again with the new departure card number, most Immigration office require a complet registration. And you day of arrival is your day number 1 and you can count yourself 90 days forward for your next 90 days inside Thailand report.

To my knowledge, the 90 day report receipt is not required to leave the country.

which is always the exactl date that you're allowed to stay in thailand for :P