You can modify the trigger frequency and threshhold, but configuring vc to be in alert status for a certain period before alerting is not possible. You can always set the frequency to be 1 minute, and then wait until vc performs the action you have specified. For instance, configure action as send email from green to yellow or yellow to red, and then configure frequency for 1 minute. That way, there will be a 2nd email at the period you want. However, now you'll get an email every minute. Not sure if this is how I would do it, but that's about as robust as the vc alerting gets. If you need something made for monitoring alerting, check out other tools, Zenoss and Nagios are two open source alternatives.
-KjB
vExpert/VCP/VCAP
vmwise.com / @vmwise
-KjB