About how long would you say Tony worked for Frank before betraying him?

I'd say about a year or so is an accurate guess. It would have to be at least that long IMO in order for Tony to be that well trusted by Frank.

My favorite line from Tony is Tony telling Frank he never betrayed him.... I guess Tony doesn't think sleeping with and then marrying your bosses wife is not betrayal.

Or cutting side deals with Sosa, when it goes against your boss' express command to do so. Nah, no betrayal at all. Fuck Tony, that's a quote.

I would say about a little over 6 months. August 1980, they do the job for Frank to get the green cards. Afterwards, maybe a week later they get another job through Omar which ends up being the Colombians. 3 months later Tony visits his family. Sometime later, definitely after new years, Tony and Omar go to see Sosa. Tony gets back to Miami and tells Frank, later that night he is almost killed.

Most of this is due to the movie looking like springtime instead of Summertime. If you pay attention to the trees, you will get the same feeling. California weather.