Practically speaking, radiation necrosis from whole brain radiation is very unusual. So most progression post whole brain radiation would be considered tumor recurrence and may be best treated with SRS. With newer immunotherapeutics, however, pseudoprogresion may be seen and needs to be considered....

I agree with Dr Chao that (barring pseudoprogression from immunotherapy), that an increase in size of metastases following WBRT is more likely tumor progression than radiation necrosis.  So there is no need to wait any specific period of time.   It isn't exactly the question asked but som...

Agreed, tumor progression after WBRT is generally suspicious for disease than treatment effects. I usually repeat  an MRI at 1 month after WBRT and reassess potential SRS lesions. If patients are on immuntherapy I would consider another repeat MRI if I am not suspicious.