The bots can exceed there knowledge limit slightly. If they have exceeded their limit they will forget their least important data over night. If they exceed the limit by 50%, then they will immediately start forgetting. You can force your bot to forget from the Knowledge page under Admin by running "forgetfullness" from the Task drop down.
They forget their least referenced data, and old conversations and context.
You can also "pin" data to ensure it is not forgotten. When you correct a response from the Chat Logs page it is automatically pinned. Also imported scripts are pinned.
You can reduce your bot's memory usage and improve its performance by disabling Comprehension, Consciousness, and Wiktionary from the Learning page (but it will affect its intelligence).