I have kept my first team and will keep it for the rest of my time playing the game. After about 4 or 5 seasons, I picked up a second team and have spent several seasons at D3, then D2, now at D1AA. If the engine had not changed, I would probably move up to D1A this upcoming season...but now I need to get a better grip on the gameplay first.
In short, do what you want to do..whatever is fun. There are big learning advantages to having multiple teams. To be "successful" (meaning deep playoff runs, and contending for a NC) you will need to spend some time, build your program, and learn how this engine works. As mentioned above, recruiting is the real key to success at any level, and is more important at each higher level (IMO)