It all depends on how flexible you are in accepting a different base when upgrading, but with the commands since this year being more seniority based than before, it is very hard to estimate.
There are in the region of 100/150 upgrades per year, but many FOs with the requirements after 2 years of commands freeze.
If willing to accept the non popular seasonal bases it could be as low as 3/4 years if with proper experience, wild guess?
Company is not expanding as it used to, and attrition is very low on the left seat.