You should be able to reach 50 jumps after some practice.
Once you hit 50 jumps, as Steven Vascellaro said in his answer, the pace will stay constant.
At this point, stop trying to time your jumps, and mash your jump button as fast as possible. This will cause Mario to do the smallest jumps and also to jump as soon as he lands.
This does not always work (I've had it fail around 70s and 80s), but after a few tries you should reach 100. More importantly, since you don't have to concentrate after 50, it will keep you from losing your mind.