The nonce in the block header is a 4 byte integer. Therefore there are only 4294967296 possible nonce values for any given header. After that, you need to modify the header in some other way to gain another 4294967296 nonce attempts. Common ways to modify the header are to adjust the 4 byte timestamp or to change the transaction list so as to get a new merkle root.
This makes sense. I guess since they're hashing the data, they could just change one piece (like the timestamp), and then they can reset the nonce to zero and start incrementing again.
thank you for your input.