I personally wait an hour, Long enough to digest and benefit from my preworkout meal, and not too soon that I will feel ill.
Post workout nutrition varies from person to person. I consume 100 grams of hi GI carbs post workout to quickly halt catabolism (muscle tissue break down) You can get this in the form of maltodextrin or dextrose. I am lazy and just buy ultrafuel, then mix some whey in with it.
There is a metabolic window of approximately 1 hour after your workout where you MUST eat your largest meal of the day. Get high protein in this meal as well. Your body needs it to regenerate and grow your muscles.