Top end 20-25, bottom end 40-50.
If you are running around at the very top level, chances are every 6-10 hours will be done and paid for by a sponsor so no big deal.
Does this give you a gain? unlikely to be notable to most if not all drivers.
I have an engine that has 25+ hours on it and compared to a new (2 ish hours from new) engine it was still very strong (seemed to give nothing away at all)
The biggest gain to be had is making sure the carby is working as it should. A rebuild kit here on a semi-regular basis is maybe the best money spent, but saying that I only replaced mine about every 12 months when I was racing local events with the odd larger meeting, others do it after only a couple of meetings.
Sorry, I forgot a disclaimer:
In this run time stated, it was mostly in restricted configuration, with perhaps 6 hours (3 or 4 meetings) of this in open format.
Also, consider how hard you're pushing the engine. If you rev it right up to the limiter on a consistent basis, 25 hours is probably unattainable for a top end.