Really can't say...
As all positions are presently filled (by incumbents) and many waiting for a chance, my best GUESS is 5 years. Just a guess. Probably more, perhaps less.
All new joiners start as S/O's, (despite the cock+bull stories of D/E hiring as Capt's and F/O's onto the pax fleet - total garbage) then promote after approximately 2 - 2 1/2 years. You remain inelligible for a base until after the "QL" check (J/FO to F/O) has been passed (about one year after commencement of conversion course from S/O to JF/O). As an aside, the (present) failure rate is about 80%. So you wait another 6 months to a year...
There's three to four years minimum. Quite a few B777 JF/O's and F/O's have quit and joined Virgin Blue and WestJet (taking a significant pay cut) from Hong Kong. I have been told by some of my F/O's that a factor (not the only factor) was the basings issue. Cathay reserves the option to promote you onto the type of their choice...there you sit for (say) three years on the 747-400 or Bus as an S/O waiting for your Oz base and wham, you're assigned a B777 course. Most would be over-the-moon happy, but the Hong Kong Factor* has become such an issue with the blokes that it becomes the last straw in a very large bundle. Bummer. Bye Bye.
* Hong Kong Factor is the accumulated B-S from an aggressive, avaricious, insensitive, belligerent and poorly informed management. When you join CX, you get two pails, one marked "$$$" and the other marked "!!!!!". You leave when one fills up. For some, the one with the "!!!!!" on has filled up rather quickly. Despite all that some say on this and other forums about it being the wave of the new millenium, there are other ways to run a company. CX's usual solution to unhappiness is to throw $$$ at it. They have yet to learn that a happy, motivated workforce can save money. Sorry about the soapbox bit, but it has to be said. "Bans" aside, your call. Think long and hard. To quote KD Lang (the lesboPETAsinger):
Anywhere but here.