I'm eventually gonna be a protection warrior. I'm specced arms/fury right now for PVE. What level will it be necessary for me to switch to prot for instances?

well i had np tanking with my arms warr in all instnaces till lvl 65,at about65-66 it gets harder to tank, (but it is still possible)anyway i would say that 60-65 is a good range to change to prot, depending on how u can manage(i would change at about 66-68)
anyway i hope it helps:D

yeah thats perfect. i just wanted a ballpark range of when to do it... like 50's 60's whatever... so thats good to know!

I was tanking ramps as a fury warrior in half tank half dps gear, after that it dependings wether u wanna quest or not. soloing while questing as a prot warrior is hard, ur gonna die alot. i switched to prot at lvl 66 and ran instances to lvl 69 and about to be lvl 70 soon. u wanna quest dont switch prot but u wanna do instances then by all means switch

Switch at 65, you can still quest, just takes a little longer to kill stuff and have to bandage/eat more.  I was DW Fury until then, but could no longer hold aggro effectively.  Plus, kept getting blown off as DPS warrior.  Everyone wants a tank, which makes getting into instances really easy now.

I am going to go back to Arms at 70 for a while to do BG's and rep grinding and then back to prot for raiding.

Yes, ofcourse you die a lot as prot

/sarcasm off
On topic, I tanked every instance til 70 included the 70 ones as fury.