If you're trying to be really exact, it will depend on what camera you're using it in. With my N80 and its automatic take-up, I barely have to pull any out of the cassette when loading the camera. Not the case with my older Nikon.

I'd give it maybe two frames on each side for my N80. But take my advice with a grain of salt as I recently packed 51 frames onto a single roll! I lost about ten of those when my camera's counter maxed out at 38 and I got scared of going much beyond that. :/ C'est la vie.