It depends.
The papers once quoted that an MB0405 can go up to a maximum of
650km b4 topping up. For this reason, most Single decks only refill
once a day when they return to depot at night. Exception tt i noe
of is svc 168, where drivers return during mid-day to refill at BN
due to the rather long, and express sector.
DDs generally return to depot sometime in the middle of the day to
refill. One can see from the driver's timetable. Its printed on
it.