seniority
5J has a seniority system in place which management respects (most of the time at least). Whenever the need arises, FOs are upgraded according to seniority provided the FO is qualified. An evaluation process (which varies according to the Chief Pilot Training's whims) is conducted then the upgrade candidates are selected based on the results.
The shortest term on record prior to upgrade would be around a little less than a year from right seat to left. At present, there are a few senior 4 year FOs who have yet to be upgraded. Most of the senior FOs due for upgrade have been with the company for about 3 years.