I could be completely wrong, but one thing I typically do while fishing is take long breaks. This seems to off-set or cool down this hidden fatigue and allow me to fish much longer. In other words, I don't think 24 hours is the reset. I think it may be much less. I've had what appeared to be a complete reset in as short as four hours.

Now it's all hidden (which I hate) so I could be totally wrong. I could have had good luck or bad luck. I could have hit some invisible reset at JP midnight. I don't really know, but I'm pretty sure 24 hours is incorrect. I'd try 6 hours and if that works I'd try 4 next time.

I usually fish about 20-30k SP and then go do something else for 2-3 hours. Then I come back and hit up another 30k. I keep this going pretty much indefinitely and I've gotten as many as 3-4 levels in a 24 hour period.