The X7 has a timetabled 30 minute journey time between the bus station and Cribbs Causeway. The handful of times I've done it southbound it's took approx 25 minutes. Though on one occasion it took just 19 minutes, that's when traffic was really light though and was pretty much greens(traffic lights) through central Bristol. I did the journey northbound once and it took 25 minutes. But a half hour timetabled journey time is sensible, to allow for any delays on the motorway. That's why for my sample imaginary timetable I gave it 27 minutes timetabled journey time.
However I do wonder on the current 309 & 310 peak time workings from Bristol can often keep to the timetables. For instance of the 1700 & 1730 depatures, they are timetabled for 17 minutes to Muller Road Top and 28 minutes to Filton Gipsy Patch Lane. At these busy times when there are a lot of vehicles on the road I can imagine these particular services easily running 5, maybe 10 minutes or more late by the time they reach Filton Gypsy Patch Lane. When I used to work in central Bristol and heading home to Horfield around 4pm-ish, I'd sometimes get the 309 or 310 and on some of those occasions by the time they got to Muller Road top they were running 10 minutes late allready. Today heading home I saw a 309(which slightly surprisingly for a weekday was an ALX 400 Volvo B7) heading Northbound at Filton Roundabout at 1805. This would have been the 1730 Bristol-Thornbury service which should have been at Filton Gypsy Patch Lane by 1758.
I also can't really foresee First running three services per hour between Bristol and Thornbury. Would be nice, but can't see it happening anytime soon. Where are First going to get the extra buses from required for this?