It totally depends on facility. Most of our new trainees spend 1-6 months as an AG before hitting the floor. This is usually varies by classroom schedules, recurrent training for CPCs, holidays, government shuts downs, etc.
Depending on how sharp you are and how well the CPCs like you, you could potentially finish all D sides in 6-8 months; I've seen some take up to 18 months to finish Ds. R sides can take anywhere from 8-24 months. Again this will depend on a variety of things from staffing, traffic volume and mostly how much food you bring in.
The quickest I've seen totally certified recently was about 18 months (that kid was super sharp) and the longest was about 6 years.