Hi Dave,
It all depends on how big the coach / double decker is. There are big variations in height and length even in the same make and model of bus.
A customer of mine has quite a few double deckers, and they range in height from just over 12 feet to just under 15 feet high. There are also short wheel base and long variants. He has double deckers that seat from 56 to 92.
In terms of length, a Leyland / Volvo Olympian is anywhere between 9.6m and 12m long. As mentioned above, the heights varied, but this was based on where they operate as some areas have lower bridges than other areas. Width wise the Olympian is 2.5m.
At 12m long, 4.6m high and 2.5m wide using 54inch media and allowing 52 inches print area, you’d use around 105m give or take on an Olympian. So in terms of full rolls you’d be right in saying 150m / 3 rolls. However double deckers of these dimensions are fairly rare so you might even get away with 100m. I have assumed you won’t be doing the roof.
Kind of showing myself as a bus nut here! Having worked on a couple of fleets locally I’ve had to work this out before :p
Hope it helps anyway