520 for bosses until Siegecrafter. Beyond him id say 530. This is orcourse with legendary meta (Pretty must for tanks in low ilvl) And ofcourse this is assuming you and your raid knows how to raid.
530 sounds good, really easy to get by doing LFR and flex for a few weeks and using all your valor wisely on upgrades so that you don't end up replacing an upgraded item too soon.
Well on stormrage the asking ilvl is 540-550 it seems....and that's for flex! Oh yea, they want people to have achieves too...really?! Flex is for those who don't want to deal with the asshats in LFR but flex is still a means of gearing up. People just want others to be already geared so they can rush through and get their shit with no concerns or cares for others....shame.