bah, email. i've had emails that have taken a week to show up (bloody tiscali and crappy servers, lol). if anyone emails me, and its important, i suggest they attach a 'request read reciept' to it- so they know i got it, and read it, at least.
i'd say, stupid internet issues excepted, they can generally expect me to read it within a day, if its at work, and if its at home, within a day or within 3 months, depending on which email address they send it to (i have a few that i just use for junk). regarding replies, if its work, i'll reply within a day- or maybe two or 3 if i'm frazzled and its not vital, if its not work, it depends how lucky they are and how friendly i'm feeling.
the last office i worked in had microsoft office outlook installed on its computer system, and it was great for this very reason. any time someone was logged on to a computer, you could IM them (i sat right next to the printer and people would IM me to ask if their really important stuff came out of it before they walked downstairs to get it, cos the printer was tempremental, lol), every email that came in for them popped up on the corner of their screen, whether or not they had outlook express running, and it did loads of other stuff too. perhaps your boss could pay out for this system- its good for organising/schedualing meetings, and checking diary stuff out, too (to see when people are around/free/available.
as far as i'm concerned, if a boss wants to be sure you've got something, and fast, he should either phone you and actually speak to you (not leave a message and assume you heard it) or IM you and make sure he sees a reply- if he has a system like that running, or even walk down the bloody hall/stairs, and speak you in person.
same with text messages. i never assume someones got my text, just cos i sent it. i used to live in a bit of a black hole/bermuda triangle, cell phone wise, and i'd get old text emssages inviting me out on friday night, on the bloody wednesday after, when i'd driven to the next town and out of the black hole.
i sometimes think that technology has made us a bit instant gratification orientated, with regards to peoples attention and response. i think that generally, if something is really vital, that someone should take a minute and actually come and speak to you about it- most stuff really can wait though, people just want your attention now cos they want it.