Whey isolate takes about 3 hrs to digest i believe. But if you eat some fats and carbs with it then it could slow down that process. Best thing to do is have a 30-40g shake within 30 min post workout. Then have a meal another 30-60 min after that. The ideal thing which i don't ever do but i should is to drink a combo shake of whey and casein. Casein will digest slower, over double what the whey will take, so you'll supplied for a while esp if its in the evening.