Whatever responses you get here, please bear in mind that nobody actually knows, although they may claim to have heard it from the horse's mouth because they know someone that knows someone that knows... I hear estimates from 4 years to 10 years+, all from well informed, reasonable (mostly) people.
The unknown factor is that nobody can truly estimate the rate of attrition, and more importantly, where in the seniority list that attrition takes place. Also, nobody has any idea how world events will affect the airline industry. So take it with a pinch of salt. One thing that I think you can be reasonably sure of is that there won't be many airlines out there where wide body command is any faster than it is at Emirates.