Whilst not wanting to divert the discussion from the primary topic which is BA Recruitment, at a rough guess it is around 5-7 years for a command. Plenty of information on the Middle East forum.
As for joining BA during your more mature years, you may be into your fifties or even sixties (if you joined in your forties) until you saw a long haul command but that is perhaps a small price to pay when comparing that to moving a family abroad with all the potential pitfalls that are associated with such a move. All you need is one of the kids to not feel settled and you either commute and send the family home or you all go back home and hope that there will be a plan C.
There seemed to be a degree of bewilderment in some of the responses questioning the logic of why one would leave BA for EK but if you are young and single, then the pros for moving have the potential to outweigh the list of reasons why you shouldn't move, providing you can see yourself being happy abroad.
It can be tough to save anything significant whilst you settle in but thereafter, it is possible to bank a comfortable amount over the space of a year at EK.