In vhl disease, can patients have several years (>10) of very slowly increasing plasma free normetanephrine into abnormal range (1.2 nmol) due to paraganglioma or does pf normetanephrine rapidly rise?

Variable: Rates are variable. The issue is when they reach either a level or a rate of change that requires intervention.

Slow is common: Elevated catecholamine levels occur only when the tumors are either large enough or active enough (critical tumor mass burden). This typically occurs slowly in the patient who is aware of their diagnosis. I advise ongoing endocrinological surveillance.