If you add an item using the indexer, like this:

Cache["TestItem"] = value;

The item is added to the cache with no expiration policy and a normal priority. As a result, the item will stick around indefinitely.
We can help the cache decide which items to toss out of memory by specifying an expiration policy and/or a priority:

Cache.Insert("TestItem", value, null, Cache.NoAbsoluteExpiration,
  TimeSpan.FromMinutes(20), CacheItemPriority.Normal, null)

Above, I gave the item a 20 minute sliding expiration. This means each time the item is accessed, its 20 minute lease on life is renewed.
I could have told it to expire after exactly 20 mins, no matter how many times it is accessed by setting an absolute expiration instead:

Cache.Insert("TestItem", value, null, DateTime.Now.AddMinutes(20),
  Cache.NoSlidingExpiration, CacheItemPriority.Normal, null)

And to really take control of the scavenging mechanism, you can change the priority of items added to the cache. Lower priority, means it is removed first when system resources are running low. The values range from "Low" to "NotRemovable". See the CacheItemPriority type's documentation for all of the values.

You will also notice that the Cache has an "Add" method. If you choose to use the Add method, adding a key that already exists in the cache will result in an exception, while doing the same with the Insert method will overwrite it. Setting a value using the indexer is equivalent to this call:

Cache.Insert("TestItem", value, null, Cache.NoAbsoluteExpiration,
  Cache.NoSlidingExpiration, CacheItemPriority.Normal, null)