1. Do you trust them?
2. Do you care about them?
Ideally it should reciprocated. The hard part is getting there, usually it comes naturally as you spend more time with the other person out of your own volition. Just because you are meeting someone everyday at work does not mean they will become your friends, do you trust them? do you care about them?
Once you friends you don't need to meet often as long the trust and care is still there. You can become friends in short time as well if you decide to put the trust early into the other person and they reciprocate. Point is, there is no formula or plan to follow.
When I say friend I mean a true friend, not colleagues or acquaintances. Were these meanings mixed into "friend" word before Facebook as well?