How long does protein powder last for once opened?

Quick question, i opened a tub of progain about 6 months or more ago and didn't finish it because i couldn't stand the flavour i'd bought. I was wondering how long it's ok to use for once the seals been opened, because i'm thinking of going back to it and just forcing it down me despite the vile chemical banana flavour.

Thanks for your help

2 year shelf life from the date of manufacturer if properly sealed and stored in a dry, cool environment outside of direct sunlight.