on a road bike its pretty easy to average 18 mph without busting a gut
PMSL
I right whatever you say.
If the peron is bike fit, i.e. used to riding then they should be able to do it in an hour maybe a bit less, but as everyone has said so many other factors can skew an average.
Tail wind of 30 mph or headwind of 30mph
Traffic
Road conditions, smooth taramc or load of bumps on crappy B road.
Somewhere on the 50mins to 80 mins scale, faster than 50 mins and you aree starting to get pretty quick and are more than an occasional cyclist, slower than 80 mins in good conditions and you should take up golf.