Waiting period depends on whether your child is in a feeder school like DESS and whether he or she is Brit passport holder.  As DESS and DC are supported by the Brit embassy, Brit nationals,  they get bumped up the list, as do the feeder school children.  Good luck with the schools, it is a perpetual and on-going problem in Doha.  They are supposed to be opening two international schools a year for the next five years, but I don't how even that will alleviate the stress on the school system.