Umm by definition push email does not have an interval it checks for mail at... it "pushes" mail instantly... your email provider has to support this though, the best free one is gmail set up as an exchange account in my opinion... any other exchange account should also support this.. as do yahoo and mobile me.
The time options are for fetch which is below the push option on that screen. As for Rogers I don't know if it supports push or not. I can tell you what I would do is set my gmail account to get the email from my Rogers account and push it to me. This is what I've been doing for years with a multitude of email accounts gmail checks and consolidates them all and even has the option to send as so email looks to be coming from them.
This is my first iPhone and it looks like I'm going to finally need to get a new email account. I've never used gmail or anything else, never needed to, [email protected] was always instantaneous, or 1 minute intervals on whatever device. Man, I've been using this rogers one for 10 years and the iPhone4 is making me need to get a new account. I thought it did everything! LOL!