Depends on what you're brewing. For me, I rarely use a secondary unless I'm adding fruit, adding wood, or aging for a long time. I typically do no less than 3-weeks in primary and then go straight to the keg. If you are really interested in doing secondary, I'd do three weeks in primary, one in secondary, and then to a keg.
The easiest way to carbonate in the keg is to just put it on gas and wait. It'll be about a week.