Your muscles will be in an anabolic state for over a week after a hard workout... They are just the MOST anabolic in the hours and days right after a workout. It tapers off slowly after a workout.
Keep eating a lot of protein all the time. In fact, since you are hardgainer keep eating a lot of food in general, all the time.
Almost all hardgainers I have met didn't eat nearly as much as they thought they did.