At the moment it seems to be running at about 4-5 years, depends on hours and training record too.
I have heard that they are trying to recruit fewer DEC's, this year the command upgrades will be approx 110 with majority on the 320. I would think also that because EK have opened up their DEC recruitment that those thinking of moving to the Middle East may feel bypass QR for the better T&C's at Emirates and the lifestyle in Dubai.