A fleet of vehicles can include construction equipment, security vans, semi trucks and a lot of other heavy and light duty equipment in between. While having company vehicles is great, the impression those vehicles give will impact your business's reputation. As such it's a good idea to keep those vehicles as clean as possible and to come up with a system for getting the vehicles washed often to maintain your presentable image.
Determine just what sort of cleaning needs to be done on your vehicles. For instance, a quick wash of the outside of a semi-truck can be done in 20 minutes or less. A full interior detailing will take longer and may take the vehicle out of the fleet for an hour or two. If no one but the company driver is going to see the inside of the vehicle, chances are the interior cleaning doesn't need to be done as often. You also need to know what sort of filth you're cleaning through... construction equipment and vehicles at steel mills are much harder to clean than vehicles which have casual road dust on them.
Determine the schedule that works best for your company. If your fleet isn't active at night, then cleaning the vehicles during the evening hours would be the best way to make sure that vehicles are clean but that they aren't taken out of the fleet during working hours. If your fleet works 24 hours a day (like a trucking company) then you might want to have a policy that vehicles need to be washed every week, or after every completed job. You should also ask yourself if the interior needs to be cleaned as often as the exterior, since the interior doesn't usually get dirtied as quickly and cleaning it takes longer. Perhaps an interior detailing once per month would work for your company, depending on the state of the fleet.
Decide if you want to have your cleaning done in house or if you want to contract it out. There are numerous services who specialize in washing and detailing fleets of vehicles quickly and on a regular schedule. If you want to do your cleaning in house then you need to have a staging area, proper cleaning equipment (a power washer might be necessary, but certainly hoses, towels, chamois, soaps and possibly wax) and employees who are ready, willing and able to clean on a schedule to keep your fleet looking bright and shiny.
Institute rules that those who drive fleet vehicles should do their best to keep them clean. Vehicles should come with small trash cans or trash bags in them so employees can clean their own messes without ruining the interior of the vehicle. Perhaps drivers should fill out a form before and after using a vehicle so the condition can be confirmed to avoid confusion regarding the cleanliness of a said vehicle.