also just for the full picture you will go from GS-X to GS-13 very quickly after that is slows down because promotions are no longer automatic you have to apply for openings and compete. the wait for GS-13 to a GS-14 is anywhere from 2-5 years, once again depending on your career field with the intelligence track being the fastest. once you get to a GS-14, 15 is again not automatic once again you have to wait for a spot, in your agency, in your career field, in your position. i started out as government employee and quickly went over to the dark side of contracting because i saw the light. imo you don't need a JD to be a special agent. you will be doing the same law enforcement as every other special agent who only have bachelor's degrees. but because you your a "fancy lawyer type" you won't get to do any of the fun stuff you will be sitting in a cubicle pushing paper.
Gwiz is right.
the average is 2-5 years depending on your background (5 is rather long and rare but i have seen it). if you have a divorce it takes longer, if you have someone in your family who is a foreign national it takes longer, if you've traveled overseas or had credit issues it takes longer, if you had got cited for underage drinking ever it takes longer.