Around 8-10 years at the moment. The numbers promoted over the next few years wuill be affected by the arrival (or not!) of the B787 and the number of retirements. There is a big bulge of Captains who will not retire for 15 years or so (ie guys in their mid-40s), the B787 will be more expensive on crews than the current arrangement and there are no plans to expand that we are aware of, so make your own comclusions from that balancing act!
As I said originally, 8-10 years is not a bad guess at the moment but as in all things in this industry, that could change either way with little or no warning.
Hope this helps whatever decision you are tring to make!