AC120-61A says in part:
The recommended occupational exposure limit for ionizing radiation is a 5-year average effective dose of 20 millisieverts per year, with no more than 50 millisieverts in a single year.
I could fly
250 8 hour round trips SDF-SAN-SDF, with 3 hours each leg/6 hours each trip at FL450, before getting to the recommended 20 millisievert limit annual limit.
Of course if you operate at higher latitudes you'll get more radiation; JFK-DXB-JVY with 12 hours at FL450 each way could be flown 40 times annually before getting to the recommended 20 millisievert annual limit.