Pitts,
Firstly, it does not matter which fleet you are on, you will get your command based on your seniority (at the moment!!) and you will change fleets if required.
At present the time to command is around 6 years, ie those waiting for thier command course this winter have been with here approx 6 years.
The 60+ age rule may prevent it coming down to the 12-18 months you mention however the expansion is expected to continue and many new recruits have very low experience levels.
My guess, and it is a guess, would be that it is unlikely drop below 5 years for the majority. I say that because the low experienced guys will be reaching the required level soon plus there will be very few retirements over the next 5 years.