Length of time varies upon alot of factors but I believe it is common to get a reply from filling in Phase 1 questions within a month with an invite to phase 2 if successful, it is then usually about another month unitl phase 3 and phase 4 for me was the following day, I believe this is fairly common. From then, if you get offered a place, it is dependant upon your situation regarding finishing work, getting your class 1 medical etc etc.
Just to illustrate this was how it went for me...
Jan 04 applied phase 1
Feb 04 attended phase 2
March attended phase 3, then phase 4 the following day.
However I failed phase 4 but got invited back to resit after 6 months. Eventually in december I resat phase 4 and passed, but it took alot of messing about to get a medical at xmas then I had to work a month notice at work so I didnt end up leaving the UK until early march 2005
so all in all for me, 14 months, minus my 9 month delay comes to 5 months, and thats not seen as particularly speedy. I think if you really nailed it you could get it down to 3 months. But again it depends on alot of factors.