I actually did some tests when I was having issues not getting any email(except for my blackberry.net address) on my phone to test it. Seemed like it averaged around 2-4 minutes with outliers in the few seconds-1 minute range and 6-7 minute range. Those were quite rare however.
All in all, it's better than my regular email client(which yes, I know I can change the time on) and has almost replaced Thunderbird entirely for me... since I'm rarely at my computer.
Should be instant if you're using hotmail, gmail, or yahoo. For gmail, I get the notification on my phone a good 2-3 seconds before it displays on gmail.com's web interface.
I have a Yahoo! account, a Hotmail account and several POP accounts from various domains and rented hosting. I rarely have had to wait more than a minute for e-mail to be pushed to me when testing or sending articles to myself from Viigo.