The American Mathematical Society has [loads of data](http://www.ams.org/profession/data/annual-survey/annual-survey) about mathematics doctoral recipients, starting salaries, faculty salaries, etc. I am not sure if the specific question about 6-years-after is answered in that data, but the [data on starting salaries](http://www.ams.org/profession/data/annual-survey/emp-docsgrtd) covers many years going back.