I don't understand the meaning of this "as full ~ as long" phrase. Is this a usual "as...as" phrase? How can I understand this phrase?
Good question. The narrator is saying that with an inbox that is "twice as full", meaning an inbox that has double the number of messages of the example he showed before, the amount of time checking it is "twice as long" meaning double the time. "Passes" here means to look through the emails (to pass your eyes over them) quickly. This phrase is being used for comparison so you can think of it as being "twice as full as before" although in this case, the "as before" is omitted because it is implied. This "as ... as ..." is not a special form or structure, it just happens to be how this narrator has chosen to make this sentence. He has skipped the second part of the usual way of comparing, "as full as before", because it is implied through the context of the sentence. Ex: This is going to take twice as long as yesterday. Ex: She ate twice as much as you.