Can daily use of ketoconazole 2% shampoo as a body wash reduce in strength during long term use as a body wash for tinea versicolor?

It's possible. I like for people to alternate therapies- letting them sit on the skin for 5 minutes or so in order to allow the medicated wash to take effect. Some over the counter remedies include selsun shampoo and ketoconazole. Oral medications can be used, but there is a risk of liver damage with those.