The Java version of DXA and DD4T use EHCache to cache deserialized DD4T object models. By default, this is a simple Time-To-Live cache, meaning that entries stay in the cache for the configured TTL and the system won't detect changes to cached items for that duration. So, with a TTL of 5 minutes, it may take up to 5 minutes till a change is visible on your website.
The Tridion Object Cache is used to cache the Tridion Objects which are obtained from the database. This cache can be configured so that cached items get invalidated as soon as they change, so that the cache lifetime can be very long without delays in item changes.
It is very common to have both caches enabled: the Tridion Object Cache to prevent unnecessary, expensive DB queries and EHCache to prevent unnecessary DD4T deserialization.
You should configure the EHCache TTL so that the potential delay in changes being visible is acceptable for your environment.
Note that the DD4T deserialization process is normally not very expensive, so if you already have Tridion Object Cache enabled, it is not such a problem if an item is evoked from EHCache after its TTL expired.