One of my players chose this as his fighter lvl 2 utility. Would this last till the end of the encounter or until an extended rest?
Sure? I thought the "5 minute" duration was meant so you cannot use encounter powers on more than one encounter unless your DM wants you to by putting an encounter before 5 minutes go by.
And to clarify, typically speaking "5 minutes" = "until the end of the encounter." Either the party takes a short rest and five minutes passes, or they don't and any effects which would have lasted "until the end of the encounter" are still active when the next battle or challenge begins.
I like this way of reading it this way. But is this rule written somewhere?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.