How long do I have to respond?
As fast as you reasonably can. Most guests are comparing several boats at once, and a reply the same day is usually the difference between winning the booking and losing it.
We understand you are at sea for most of the working day. The evening after you come in is fine. Three days later is not — by then the guest has booked someone else.
There is no fixed deadline after which a request expires, but there is a practical one: guests booking from abroad typically send two or three requests in the same evening and take the first captain who answers. A request left for more than a day is usually already lost. If you know you will be out of signal for several days — a multi-day charter, a haul-out — close those dates in your calendar first so no requests arrive to go unanswered.