I try to expand as quickly as my fleet can support it (based on using a cap ship early to maximize exp) and as my empire can afford it.
I might sweep a neutral system for exp, then let the cap ship sit awhile and regenerete shields/hull...but won't colonize it until I can afford the 2 upgrades to remove underdevelopment costs.
Seems like once you get 4 or 5 planets, you can pretty much afford to expand as fast as your enemy will let you.