How much time is required to pass CCIE Lab exam, if you have already passed CCIE Written exam?

it depends upon the track and also on your knowledge. As we know very well ccie exam has a time limit and each section has its own timings so we just don't need knowledge , we also need good hands on and troubleshooting skills. 
if your knowledge is good and you have good hands on on some major topics like for eg. if we are talking about ccie routing and swithing , i will say you should be good with mpls ,dmvpn ,eigrp ,ospf and bgp . then 45 to 60 days are enough for practice . yes not only labbing , you have to work on tshoot as well as diagnostics too. 
so on an average 2 or 3 months if you are free and lab is the only motive  , else it varies on a lot of things like timing , skills and study circle.