End game raider here I can tell you some fights are hard to the point that a month or two will be required before you can finally down the fight. If a fight is particularly hard it will take 3-4months of effort. T13 and the like will require serious investment in time and effort. And you'll be required to push your class to its absolute limit weither that be aggro generation and cooldown use as a tank or healing effectively or just plan putting out the required dps.

I should make a note that right now at the time of this post there is no raid right now that is current due to heavensward being just released but rest assured in the following few weeks content will get much harder with the new raids being released.