Generally I recommend about a year to my clients. The tubes will wear and drift as time goes by, they will never be as bad as before your first calibration, but they will not be accurate. This is just like tuning up a car every so many miles. I have customers that call me in every six months to insure that their displays are operating to the standards, but most of my clients wait about a year for their follow up.
I had an ISF calibration on my RPTV about a year and a half ago. If my brightness hasn't really changed since then, does that mean I probably don't need an ISF touch up? My set is 3.5 yrs old now.
I had mine re-calibrated after just about a year. I thought everything was perfect, and could not be improved upon. But this was to be a free-bee, so why not?
I was wrong. Wrong. Wrong.
Grayscale had drifted a little, and convergence was off(I had been touching it up, but Kevin did a slightly better job).
I swear it looks better than after the first calibration.