In TUI (Thomson) it has been around 10-15 years. It looks like that will drop down to about 4 years though for those to be upgraded in time for summer 2019.
The aviation industry is going through the best of times and 4 years to command in TUI is about as good as it gets! If you remember that the airline industry is notoriously cyclical then those joining mature airlines like TUI now, may we’ll be joining at the top of the wave and end up waiting a long, long time before their turn comes around. I know - it happened to me!🛫