It should be instant. I get my POP3 emails off various accounts in less than 30 seconds. At most it makes maybe 45 secs to 1 minute. RIM uses push technology, unlike Apple or WinMo, so once an email hits your inbox it's automatically pushed to your phone instantly.
"Instant" is impressive.
I understand that Verizon 'pushes' the email to the phone and that should be quick.
However, POP3 email services do not 'push' data to Blackberry servers nor even offer notification services. To accomodate POP3 mail accounts, Blackberry servers should have to poll your POP3 account.
I'm surprised they have the bandwidth (and that all the POP3 servers allow) to continually monitor that many email addresses!!
However, the proof is in the pudding as they say...