Level to spec full endgame prot?

My next respec will cost 15g, so I want this to be the last time I respec until 60. =)
Anyhow, I'm currently 48, and my goal is to tank in the engame. I've started to run into resistance from PuGs for places like Mara for tanking as a Ret spec, and would like to know when the appropriate level for a future endgame tank to go prot is.
Duki, I already know where you stand, and I would like to milk my Ret spec for soloing as long as feasible. That said, I prefer instances. Theloras or Jcarrilo, if either of you wanna chime in, 'twould be super.
Anyhow, I'm currently 48, and my goal is to tank in the engame. I've started to run into resistance from PuGs for places like Mara for tanking as a Ret spec, and would like to know when the appropriate level for a future endgame tank to go prot is.
Duki, I already know where you stand, and I would like to milk my Ret spec for soloing as long as feasible. That said, I prefer instances. Theloras or Jcarrilo, if either of you wanna chime in, 'twould be super.