When should you discontinue narrow band UVB when a patient is in clinical remission from patch stage mycosis fungoides?   

It depends on the patient, and whether they have a history of flaring after stopping phototherapy. In that setting, maintenance phototherapy (once weekly or sometimes every other week) may be helpful.

MF treated TIW until clear then weekly for maintenance.

NBUVB usually needs to be continued for long-term maintenance. How often depends on patient, but typically weekly treatment will work.