As many times as you like. There's no limit. Of course you can "optimize" not to subdivide to a ridiculously high level, if it doesn't start to look any different, 'cos it's a bit lighter to export and use for baking.
At most, polygons should not be smaller than what a single pixel will be on your baked textures. It really depends on how dense your model is, sometimes 2 is enough, sometimes 5. And sometimes different parts need subdivided more, like a tiny screw versus a large smooth curve.
For characters in ZB mostly 6 subs are enough. Occasionally 7. Thats comming from a single div with zspheres. Hard surface stuff usually 5 sometimes even 3 or 4!
For a polygonal hardsurface (mechanical) high frequency mesh object and/or shape, applying a sub-surface modifier at level 2 with shading set too smooth seems to be sufficient IMO.
In case you're rendering animation (and as such you divided it in shots), you pick the lowest level that you can get away with for the moments of the shot where the subject will be the closest to the camera. If it looks good when the subject is at its closest to the camera, then obviously it will look good when it's further away. When the shot has the subject always far away from the camera then you can use a low subd level to speed up the rendering.
Even if the program let's you change the subd level during the animation, you should probably use the same subd level for all frames of a single shot in order to avoid some popping or unexpected result.
in the old days where my workstation had only 4gb of ram i could only use 7mio quads in mudbox... i used a subD table to get the polycount as close to 7mio as possible für a human body...
now i dont think about how much poly i have... if i like i go up to 60mio or more...
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http://wiki.polycount.com/wiki/SubdivisionSurfaceModeling#Primers
When the shot has the subject always far away from the camera then you can use a low subd level to speed up the rendering.
Even if the program let's you change the subd level during the animation, you should probably use the same subd level for all frames of a single shot in order to avoid some popping or unexpected result.
i used a subD table to get the polycount as close to 7mio as possible für a human body...
now i dont think about how much poly i have... if i like i go up to 60mio or more...