How many enquiries actually come in after hours?
Published figures put it around 65% of website enquiries arriving outside business hours, and most jobs going to whoever replies first. The number that matters is your own, and it is in your analytics.
The industry statistics are consistent enough to be worth knowing: roughly two thirds of website enquiries land outside business hours, something close to four in five jobs go to the business that responds first, and the average business takes around two days to reply. We cite the sources for those on the home page rather than asserting them.
What none of that tells you is your own split, and you can find that out for free. Open your analytics, look at when sessions happen, and compare it to when your phone rings. The gap between the two is the shape of what you are missing.
Be careful with the framing, though. An after-hours visitor who leaves is not necessarily a lost job — some of them come back in the morning. The honest claim is narrower: the ones who do not come back are invisible to you, and there is no way to count them from the inside.
That is the case for answering at eleven at night. Not that every one of those visitors was a customer, but that you currently have no way of knowing which ones were.