The larger the map, and the fewer the enemies, the longer you get before full scale battle becomes a priority.

The more enemies, the more likely you will get one very close to you, and you will ahve to concentrate on defenses.

Ideally, build you own map like I have, with few phase lines, so that you can see where they start from when you view the built map, and make sure that you can get a dozen planets colonised before you even come accross the first enemy scout. Then you dont have to worry about defenses, and can concentrate on research. Also, if you ensure that none of the enemies are aggressive ones, the time before contact should also be longer.

The longer you can hold off first contact, the more coloninising and research you can get done. Once you start building defenses, your resources get drained. So its also an idea to fix the teams before starting, and select no pirates. That way, you dont get the missions thrown at you and dont get pirate retaliation when you fail them. Combined with few phase lines, this means you only need defenses in key choke points, and therefore dont need to waste money on defenses at all other planets.