So a reasonable response time must be a part of the offer.
What is a reasonable time? It depends.
In my opinion reasonable is usually 24 hours or perhaps 48 hours at the most. In today's high tech world obtaining an electronic signature is a matter of hours, not days.
Still there are times when a reasonable response deadline may be several days if the sale involves unusual circumstances (absentee owner, divorce, estate sale, etc.)
The time allotted for a response to an offer is really up to the prospective buyers. It is their offer.
Sometimes it is a tight schedule, a choice between multiple equally desirable properties, or some other reason that drives the response time.
In most real estate transactions time is always of the essence. And that starts with the offer!