How often should air spring forks be reinflated (topped up)?

Forks that use an air spring, as opposed to a coil spring, have to be pumped up to the desired pressure to work as intended (using a shock pump). From experience, I know that they lose pressure over time. However, I usually notice the pressure drop only after it becomes very apparent, and the time in which this loss occurs seems to vary somewhat, so I do not know what is the normal interval for reinflating forks, or the normal range for such interval. The service manual of my forks (all are RockShox) does not state this interval explicitly. Rather, it describes intervals for more elaborate service procedures (50 and 200 hour service) that involve deflating and reinflating the air spring as an inevitability, so I do not think that they should necessarily be taken as reinflating intervals.

I would like to know this primarily because it would give me an indication of whether my fork is damaged or in good condition (I can always check if my fork is losing air, but without knowing how often this loss normally occurs I cannot tell if the fork is fine or not). So, what is the normal fork reinflation interval?