One thing I'd like to know, is what is the maximum length of time a bus spends out on the road? I can imagine 48 hours perhaps?
Do you mean before it needs re-fuelling?
I'd say probably about 18 hours on average, although the fuel consumption will vary by type of bus and operating environment.
A few years ago, I lived in a shire county that had put on quite a lot of evening services with rural bus grant / challenge money - in some cases, the company that ran the daytime service won the tender for the evening service, and kept the same buses out all evening.
There were a couple of routes where it was touch and go whether the last bus would manage to get back to base without running out of fuel, and it was not uncommon for the late shift engineers to have to go out and rescue buses. I think mid-evening bus change-overs got written into the schedules eventually...
Then there's the practicality of cleaning buses.
I don't work for a London bus company, but I'd be surprised if buses that have been out all night stay out beyond the end of the morning peak, they may even call in for a refuel and clean just before the morning peak hits full swing. Likewise, I'd be fairly sure that buses that go out on night duty have been in for a clean and re-fuel during the evening.
I don't think there's any London routes that have their detailed (bus) schedule on the internet, so can't prove it. It's also worth bearing in mind that a lot of London routes now involve buses running 'private' to / from garages for driver change-overs, particularly at odd hours.
The growth of the London night bus network is quite a success story - I can remember the night bus output for the whole of London being perhaps about 40 buses, mainly geared towards the need of print workers in and around Fleet Street, and, with one or two exceptions, not running on Saturday nights. It all started to grow in the mid 80s and then someone noticed that there was a demand for travel on Saturday nights...