It depends on what you want to do and see. I wouldn't say that I lived there, but I was there for 5 months. The longest period of time i was in one place was 2 months. Honestly i don't even know where to begin describing what it was like. it was without a doubt a very positive, but sometimes stressful, experience. I think that everybody should travel to India at least once.
I traveled only in the north, so that is the only area i can really give advice on. For the south i heard that Kerala, Hampi, and Pondicherry were the places to see, but I couldn't tell you if that is true.
To start with let me say that traveling in India is not as daunting as it seems. i started out with a program, but stayed on after its end when i realized that i would be able to get around on my own. unless you have about a year, you will probably not see everything, so don't worry about seeing everything. You can probably get by just fine with only English, but it never hurts to know a little Hindi or whatever the local language is, if only to see the shock on people's faces. lol. The cost is very cheap, you can easily get by on $10 a day (room, board, insurance, food, and maybe transportation). Sanitation is a big problem over there (to us!), but if you just use common sense you will be fine. it is a very safe country for the most part, but if you are a lone female you do need to exercise some caution. nearly every female traveler i came across had at least one complaint of minor harassment.
the places that i saw that i really liked and/or you have to see are
- in the north: Ladakh, Dharamshala, Manali, Kashmir are all in the mountains and are much more relaxed than the cities in the central plains, although Kashmir is not so safe in Rajasthan: jodhpur without a doubt, Jaipur is more popular with tourists but i didn't like it as much
- in central India: Agra of course, Orchha (near Jhansi), Varanasi (stressful, but worth it for the sights and culture shock), Khajuraho (although they really go after tourists here), and Delhi of course (but it can be a bit stressful if you stay there too long)
- in the east: Darjeeling is a good place to chill, and i heard Sikkim is amazing
the best advice i have, if you haven't already done it, is to get a guidebook (i used lonely planet) now and start looking at pics and reading about the places. i really hope that you get as much out of it as i did.