As in all cases, the time frame must be spelled out in the confines of the purchase contract. If the seller needs more time he needs to put it in writing in and amendment agreed to and signed by both parties. If not the contract will have dates that expire and the contract is over at that point. Ask your Buyer's Agent where you stand at this time. We can' t know here on this site without reviewing your contract. And of course that is not our job, but the responsibility of your agent. Good Luck.
Your Castle Realestate
If the buyer has formally requested that the seller make repairs on the septic system as a condition of the purchase, I would say that 3 business days is a reasonable time to give a response. However, if this is a condition of the lender or a term of the contract from an Amendment to Address Concerns, then the Amendment would dictate the answer come prior to the end of the Due Diligence period. In this case, I would ask your buyer's agent, if represented, or the seller's agent, if you're a customer, for an answer TODAY! Time is of the essence in any real estate contract. Feel free to provide more details about the timeline of the contract.
"We're about building relationships, not transactions"