It all depends on the tribe.
Teutons - You can immediately start building barracks, troops and start raiding on the v2 so it can support itself pretty much on the go, so you should be getting it pretty much the earliest. Once you have about 100 maces, it takes 4 days of nonstop (and I mean nonstop) raiding to get all the infrastructure and etc going. Given you'll spend your first day in BP and raid for two days to build up your mace count to 100+, you should be looking at earliest 7 days and 8-10 days if you want to do something else in between (like getting paladins, fighting other Teutons, etc).
Romans - They can too support themselves with some raiding, though not as profitably as Teutons. You should probably be spending your first week on getting EIs and pumping them out and then devoting next week to getting your new village. You can however choose to pump out as much as troops as you can on first week, then build buildings / resource fields on the second week and get your new village mid third week.
Gauls - Slowest to start with, it speeds up once you get TTs. You should raid for TTs and then raid with TTs for more TTs. At around 60 TTs, I start working fields up to 4-5 and then getting the new village. It should take about 15 days from the first day for you to have your second village.