You can look at the current activity charts and extrapolate from the particle flux, but it's likely going to be below 1mSv for a 6 hour flight. The XKCD radiation chart provides the most sensible comparison for most people I think:

https://xkcd.com/radiation/

As a comparison, a typical flight might only have a dose of 40uSv, so like 25 normal flights. But still many people take many more trips than that.