"...I like that you call it an Allison and not RR...." Heck, I can remember when it was Detroit Diesel Allison.
Seriously, I can't quote any figures but the failure/shut-down rate must be very low for a well-maintained engine - do regular compressor rinsing, regular engine washes (chemical washes) and keep the fuel nozzle clean. Perform regular power-assurance checks to ensure everything is good and it will give you many hours reliable operation.