Depends on where you're at, what school, all that. I've heard the average is about 4-5 years, which is why Judoka shouldn't take offense to people saying Purple Belt in BJJ is equivalent... They're talking about average training experience (with the oft innacurate "2 years per belt" BJJ average that's slung about).
I've done both, for a considerable amount of time, and I have to say that the best benefit was the ego boost. Ha ha... In my last Judo school, they called me "the Jiu-Jitsu guy", and I would win the majority of my tourney matches by sub, and often hear "Yeah, his Jiu-Jitsu is REALLY good...". Then, I got to my new school, where they do BJJ and VT classes, and I was asked my opinion about a throw we were going over (O Soto to Uchi Mata, one of my favorites, they were asking because they knew I'd done Judo a long time), and I demonstrated some of the little pointers I'd picked up from some great Judo BBs I've trained with, and everyone was like "Well, you're like a black belt, right?"
BJJ- a low level Blue
Judo- a low level Brown
In my head when I hear stuff like that- 90th Dan Super Ninja of Matwork and Throwy Stuff
My first experience with either art was in a mixed club that did both... No differentiation was ever made (I just made up that word, cause I'm a super ninja), but most of the guys just called everything on the mat JJ, everything standing Judo, and we'd have some techniques from both ranges, some Judo and JJJ katas, and a lot of live rolling. Only problem was, when I got to a strictly competition oriented Judo school, I had a lot of adapting to do to limit my reacitons to Judo-legal moves (my favorite technique from bottom half guard was a sweep to knee bar, I also love foot attacks... etc). Now that I'm doing BJJ two nights a week, my hips are good, and I can own a position when I get to it, but some of my movements aren't as fluid, and I find myself defaulting to kesa gatame a lot, and having the better BJJers escape to my back (why did it always work in Judo!?), and having guys pop out in no gi like nothing. I finally adapted to the BJJ/Sub Grappling kesa, with the underhook instead of the headlock, and I'm just trying to readjust to the fluidity and the other options you've got in BJJ. Conversely, I think I'd smoke the competition at the Judo tourneys I've been in, because my ground game has already improved greatly.
Do both, but only compete in one per "season" or something, or you'll just confuse yourself and hamper your development in both.