It's not very probable at all, but you have a non-zero chance of hitting a block, same as every other miner out there...

For example, another user here, Sledge0001, hit a block with one of his USB miners, which in total were hashing at 8.3TH/s: https://bitcointalk.org/index.php?topic=5237323.msg59084637#msg59084637

You can read more about solo miners finding blocks here: https://bitcoinmagazine.com/business/how-small-solo-miners-solve-bitcoin-blocks