How this card actually work?

Does this means that after i have been cleared guest like 8/8 guest points, then i have to discard this card, or does this means that after i have been committed to guest and final result has been done, then i discard this card?

I really need answer this as soon as possible

"resolve a quest" just means finishing the questing phase -- calculating quest points, then placing progress tokens. So if Escort of Edoras commits to a quest, he will be discarded after you place progress tokens, whether you clear the quest card or not.

Oh... well thanks for answer

I sense that you are not fond of the answer. At face value, it may seem rather silly: pay two resources so he can quest once and lose the ally?

However, this is one of the characteristics of the Rohan trait. Rohan characters like to leave play after they have done something. The key is building synergy around this. This is exemplified in the new Hero from VoI, Eomer. He gets +1 ATK when a character leaves play. Other cards have benefits too: Prince Imrahil readies when an ally leaves play and Horn of Gondor adds a resources to a hero when a character leaves play.

Also Escort from Edoras can be a good support ally in those quests that use willpower for various challenges, like escape tests in dead marshes and hide tests in black riders.

When the time comes he can then be sacrificed for a power questing round or to a big enemy as cannon fodder (he is one of the few allies that can legally block witch king after all; not that there aren´t other ways around him)