email etiquette

a long long time ago, when the internet had just be invented by al gore, before the boom of p2p and just in the infancy of napster, email was my communications method of choice. i was delighted with every message i received and replied to emails right after i read them. however... i soon learned that no one would reply to my instant reply emails. at least not instantly. so i decided to reply to them later on when i had more time. only sometimes i forgot and never got to reply. but what exactly is the proper time-frame for replying to emails as dictated by email etiquette? is it rude to reply to an email from several months ago? or is it better to just simply pretend like it was lost in the mail? or even more importantly, is reply-time inversely proportional to the amount of regard you have for that person? perhaps some experimentation is in order.