Something I'm Very Curious About

When people email you, how long does it take you to get back to them? Perhaps it's different depending on who emails you, yes? Or how busy you are or how accessible the internet is?

So, what if it's a good friend of yours?
-or a less close friend, or an acquaintance?
-or personal emails or business/work emails?
-or news letter or prayer letter where a response isn't necessary?
-or an email you know is going to take a lot of time and effort to write and you don't have a lot of time.
-or you don't know what to write?

Do you put emails off? Is so, why and for how long?

I really don't understand myself in regards to this issue (although others have tried to psychoanalyze me about it--you know who you are :) Maybe your responses will help shed some light.