Sorry - at the moment, there's no way to adjust that.
Curves need a finite max for a number of reasons, but there's nothing magical about 30 minutes - it was just a nice round number that I felt offered enough "headroom" when designing things.
I looked through my code last night that uses that max number, and, at first blush, it might be fairly straightforward and safe to include a pref setting for adjusting that to longer times, then adjusting the areas that use that number to use the pref setting instead. I'm not sure if I could allocate the time in the next significant update to do this, but send me an email if you'd be interested in testing that feature. I'd be happy to work with you on it if I'm able to tackle it in the future.
