When to switch to protFollow

k, Dumb question that I'm sure will get me yelled at, but my internets are slow and I can't do all the reading I would like.

I plan on lvling in the arms tree trying to get to MS. I'll be attempting to act as a MT in instances as far as I can go without being prot specced.

Question...At what lvl does a warrior have to switch to a prot, or at least a MS/prot, spec to be a viable MT. I've heard that around 50 you'll no longer be able to have success as a MT without speccing prot or at least a balanced hybrid.

Also, if anyone would care to list a build that I should aim for at that lvl...that would be great.

Thanks for your help guys.

lvling my second warrior to 70 (long story) and i think its fair to say u dont need a full prot build until 70. my firsy warrior i was fury up until 70, then went prot for raiding. i tanked fine in instances until around 68 where i remember having a bit of trouble. the warrior i now use is mostly arms. im at 65, i have filled out the talents i like in arms, 5 points in fury for crit chance, and the last 10 points in prot for increased armor and defense.
my advice wood be to continue in arms/fury until u feel like ur dps is wat u want it to be, and if it cant get much better and ur having some trouble tanking put the remianing in prot. however, even in the outlands u dont need a full prot build to tank the regular instances, from ramparts up to w.e
the only reason id go prot wood be for raiding or doin heroics at 70, which i will be.