No commenting on the OP, but not all riders are created equal. Someone with natural talent can accomplish far more in the same period of time than someone that struggles. You can't really assign a time line to how far and fast someone should progress. Honestly, I think the trainers that stick with those who have a hard time with simple riding basics (and those riders are absolutely out there) are saints.
In 10 one hour lessons I expect all my pupils to be able to canter. My own school which I had for nine years we had 300 riders per week coming through and not one was unable to canter in that time frame.