...well I got promoted to a first degree brown (last one before black), and i've been training for 2 1/2 years now. by the time I get black, It'll be 3 years. is that a decent amount of time to earn one, or is it too early?
depends on the style, what other martial art experience you have and how often you train. also, don't think about black belt being the end of your training. in most respects it is just the beginning.
Hours not years Someone could have simply trained twice a week for an hour and a half for 10years, some one else could have done the above plus an hour or two every single day by himself for 5 years. Who's put the most into his art, it does take time for things to sink in but intensity of training is important. Concerntrate on the hours you put in and not the years!
Stop concentrating on time put in and start concentrating on the quality of your training time. If you are training BS for 10 hours a day for 10 years all you've done is waste your time. If you train with quality for an hour a day for a year you'll be better than the guy who trained for 10 with nothing but BS in his program.
IMO, once you get to BB in an art, you're not an expert, you're a serious beginner. You have some of the basic techniques and have performed them to an adequate standard, but to progress, those basics need to be excellent and you'll be learning new techniques.
Depends on your art. Some arts see black belt as the gateway to the important stuff - master of the basics, almost. 3 years might not be too short for that. In other arts, BJJ for example, a black belt means much more, and really means you are an expert in the system.
To actually answer your question, yeah, 3 years is pretty standard for arts like Karate, Tae Kwon Do, the like. My school the average is around 40 months, very very few can do it in 2 years, most take around 3-6 years. Generally serious adults take 3 years or so, while serious kids and not so serious kids take longer. For example, when you start at age 7, it is unlikely (just based on your average 7-year-old's understanding and attitude) that you would get a blackbelt in 3 years. Always remember that a black belt is just a title. If you've worked for it, it means something. If you haven't, it doesn't. I can go on eBay right now and buy a belt that is black. Doesn't mean I'm a blackbelt. Also, they didn't even exist until the late 1800s, so don't think that there's some ancient criteria for blackbelthood.
Don't get to hung up on ranks and belts. I have seen white belts with no experience run rings around "Self Assured" black belts simply because of motivation and natural aggression. I have also heard many tales of very skilled martial artists being beaten by street thugs with no MA training. The point is a rank or title only has meaning when you are in a class for the style of which the rank or title was given. Outside the dojo you are nothing other than what you are physically capable of achieving, as in most the people you may fight will not care if you say I am an "Extra Dark Black Belt with Fiddy Billion Dans". Focus on your abilities physically, mentally and spiritually. Rank will follow but let ability lead you!