I'd suspect the more exercise you do the more that number moves up, you'd be breathing harder. I'd hesitate to try and put a 'hard limit' on anything like this.
The normal expectation is maybe 10-25% change. Any more than than would require things likes 8 hours of exercise per day.
http://www.dtic.mil/dtic/tr/fulltext/u2/a240839.pdf
I will update this if I find it again.
[0] http://marathonswimmers.org/forum/discussion/90/when-to-gain...