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.