Frequency of a CLA will depend on amount of use (or disuse) and exposure to dust, heat, moisture, type of handling, etc. In general, if it ain't broke.... I have an M6 classic from 1995 that has never had a CLA, and it is operating in tip-top shape as far as I can tell.
On the other hand, since it is already going in for the viewfinder update, I'd consider at least a partial CLA, which DAG offers. Ask him about the finer details of the 3/4 CLA.
Btw, what does a viewfinder upgrade cost these days?