It appears to make little difference to the end result of a light treatment whether you illuminate vitiligo spots two or three times a week. 

In the past, light therapy with UVB light twice a week was the starting point for people with vitiligo. This has now become three times a week. This is possible because many people nowadays have lighting equipment at home and no longer have to come to the hospital every time. But does lighting three times a week have a greater effect? This was investigated by physician-researcher Sanne Uitentuis at the SNIP, the Institute for Pigment Disorders in Amsterdam.

Rather repigmentation
Uitentuis looked at how the patients themselves assessed the results after the treatment, then and now, and found that there was no difference in the end result. However, something has changed in the treatment: nowadays patients are now routinely advised to use certain ointments (fluticasone and tacrolimus) in combination with lighting. According to the SNIP, repigmentation now starts a little earlier than before and that is why we stick to lighting three times a week (at home) in combination with ointments.

The research was conducted among fifty participants, which is too few people to come to a scientifically sound conclusion. This requires a larger study involving more people.
Source: Spotlight no. 3, 2018, the members magazine of Vitiligo.nl