A lot depends on the species and your supplementation schedule. Some
Species can do ok without any UVB for 6 months out of year, when housed outdoors during warm weather.
Vitamin D is fat soluble, which means that D3 is made upon exposure to sunlight or UVB and stored in the organs and tissues for use. This means they don't require daily exposure for hours and hours and 20 mins outdoors, 3x a week would be a good start.
I will caution you that if this is a cost saving measure, chameleons are not a hobby to cut corners are and I would have a UVB bulb ready for times when your schedule is less predictable and the weather is not suitable. Chameleons appreciate and are adapted to receiving varying amounts of UVB throughout the day, so why not do your best to replicate this in captivity by supplying a healthy amount at all times?