Apparently its completely random.

My rigs typically return shares from a few 1000's to hundreds of millions and on occasion a few in the billions 11.5B 19.2B 49.9B have been my rig's all time best shares.

I've yet to see anything in the Trillions region and my system has been running for over a year.

I've just set-up a new BitAxe Gamma 1.1TH/s and the best that's managed in the past week has been 98.8M

But like I said, it means absolutely nothing and the only number that matters is if it exceeds the network difficulty, then and only then will your miner mine a block and receive the block rewards.

Such is the nature of solo mining.