You probably need at least 5 months to establish yourself as "Must See TV" on RAW, appear on mulriple PPV's with at least one major match on one of the Big 4 (Rumble, WM, S-Slam, S-Series), and make your way through the house show circuit.
As has been discussed in various posts over the past few years, the days of longer reigns, like Hogan near 5 year run, Backlund's 5 plus year run, Sammartino's incredible 7 year run (followed by a 2nd reign that lasted about two years) or Flair with a whopping 5 separate reigns each lasting 13 months or more, those days are gone. The TV is so much different now, with weekly live shows and monthly PPV's, for most of the 80s een with wrestling all over local and national TV, none of the major promotions did more than one or two big shows per year and rarely showed top level matches on free TV. WWE did maybe 3 or 4 Sat Night Main Events, the NWA would do one or two Sat Night SuperShow events in their weekly Sat evening TBS slot, eventually transitioning into an occassional Clash Of Champions event. Excluding the other promotions that ran durng this time, in terms major events, pretty much all you had was WrestleMania & Starrcade (which actually came first, the massive success of which prompted WWE to create WrestleMania). By the end of the decade Survivor Series was gaining credibility as a must see event, something The Great American Bash already had. That was it, even SummerSlam didnt debut until 1988.
In that era champions held titles longer because you did not see them wrestle as much, at least against top competition, unless you saw them on the house show circuit. Even then a guy could appear in your town 3 or 4 times and be a draw each time, it was the only time you saw him wrestle against his arch enemies and top contenders. Now, chances are you'll see the top challenger face the World Champ as many as 3 or 4 times just on RAW, let alone at least two PPV's. With such an expediated schedule feuds move faster making it tougher to keep champions viable longer. Unfortunately faster changing titles makes it seem less special when a belt changes hands. I cant even remember half of Cena's or Edge's title losses, but even after twenty years I remember every time Hogan or Flair dropped a title in the 80s, or Backlund's historic loss to Iron Sheik.
To that point I'd say 5-6 months is probably good, unless someone is really over and has significant potential, then ride it out for awhile, making it that much more special when they finally do drop their title.