How long it takes to load an interstitial is network dependant so it varies I'm afraid. I couldn't find any specific information on how long you can keep an interstitial loaded, but you may find some of this useful (taken from the admob getting started guide):
Consider whether interstitial ads are the right type of ad for your app.
Interstitial ads work best in apps with natural transition points. The conclusion of a task within an app, like sharing an image or completing a game level, creates such a point. Because the user is expecting a break in the action, it's easy to present an interstitial ad without disrupting their experience. Make sure you consider at which points in your app's workflow you'll display interstitial ads and how the user is likely to respond.
Remember to pause the action when displaying an interstitial ad.
There are a number of different types of interstitial ads: text, image, video, and more. It's important to make sure that when your app displays an interstitial ad, it also suspends its use of some resources to allow the ad to take advantage of them. For example, when you make the call to display an interstitial ad, be sure to pause any audio output being produced by your app. You can resume playing sounds in the onAdClosed() event handler, which will be invoked when the user has finished interacting with the ad. In addition, consider temporarily halting any intense computation tasks (such as a game loop) while the ad is being displayed. This will make sure the user doesn't experience slow or unresponsive graphics or stuttered video.
Allow for adequate loading time.
Just as it's important to make sure you display interstitial ads at an appropriate time, it's also important to make sure the user doesn't have to wait for them to load. Loading the ad in advance by calling loadAd() before you intend to call show() can ensure that your app has a fully loaded interstitial ad at the ready when the time comes to display one.
Don't flood the user with ads.
While increasing the frequency of interstitial ads in your app might seem like a great way to increase revenue, it can also degrade the user experience and lower clickthrough rates. Make sure that users aren't so frequently interrupted that they're no longer able to enjoy the use of your app.