The honest answer is: however long your product's label says, and it's different for every product. Some wipes claim one minute, some sprays need ten. The number is printed on the container, and it only counts if the surface stays visibly wet for that whole time.
Clean first, then disinfect
Disinfectants don't work well through debris. If a surface has visible blood, saliva, or material on it, clean it first, then apply the disinfectant for the contact time. With many products that's two passes: one wipe to clean, a second to disinfect. The old 'spray-wipe-spray' method does the same job with a spray product.
If the surface dries before the contact time is up, reapply. On a warm day with a fast-evaporating product, that happens more than people expect.
Barriers change the math
Surfaces you touch a lot during treatment (light handles, chair switches, the keyboard) are easier to cover with a barrier and change between patients than to disinfect every time. If a barrier was intact, you replace it; if it was torn or the surface got contaminated, you clean and disinfect before the new barrier goes on.