How long does the seller have to respond to an offer, if there even is a time limit?

If it's a Written Offer, it ought contain a "Open for Acceptance Clause"; otherwise Case Law in your State may have set a standard. Unless you're accepting the Offer "exactly" as presented, it doesn't really matter when it would expire, because you'd be changing some part of it anyway. If it is just a Verbal, then it's good for as long as it takes the Buyer to change his mind.

IMPORTANT NOTICE: This information is provided as basic educational information by the author and is not a substitute for the advice of an expert and/or the advice of a Lawyer. There is NO representation as to legality, accuracy, correctness of the herein information and the reader is strongly urged to consult an Attorney in the relevant jurisdiction to ensure accuracy before acting on this information.

Exactly! There will rarely be a time when both parties are willing to accept the contract exactly as is, so each time the contract is changed it is essentially presenting a new contract. However, I suspect there is established case law that would take effect to prevent sellers from holding onto contracts for unreasonable amounts of time before accepting.

Local preprinted clause in our standard "Agreement of Purchase and Sale" (Offer)as follows:

1. IRREVOCABILITY: This Offer shall be irrevocable by .............................(Seller/Buyer)
until ................. a.m./p.m. on the .... day of ........ 20.., after which time, if not accepted,this Offer shall be null and void and the deposit shall be returned to the Buyer in full without interest .

Important Notice:This information is provided as basic educational information by the author and is not a substitute for the advice of an expert and/or the advice of a lawyer. There is NO representation as to legality, accuracy, correctness of the herein information and the reader is strongly urged to consult a lawyer in the relevant jurisdiction to ensure accuracy before acting on this information .

In my neck of the woods, most PA's in this market don't contain a irrevocability clause, rather "time is of the essence" which implies that an offer should be addressed as quickly as reasonably possible.