Once iAd is displayed, does it never go away?

Is it okay to display and hide an iAd at will? Or 开发者_运维技巧is there an Apple-policy that says we MUST show an iAd at all times?

Also, any way to configure (or if its even possible) to set the interval at which the ads are rotated/updated?

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The iAd Programming Guide is quite specific about this:

Once you display the ad banner, you should leave it visible for at least 30 seconds, unless the user requests a new page or screen as described in your agreement under “Minimum Rendering Time”.

I personally haven't signed any iAd agreement but if you use it in your app, you presumably have. So look there for more details.