How do I set my default search to 365 days instead of last visit when someone searches on my forum?

Please could you clarify what you mean here? I ask as search itself would not look at items since last visit, but would actually look for things on your site as a whole. Are you referring to a stream perhaps?

Yes, that's what I meant, the stream when you click unread content it only shows last visit instead of 365 days like this forum.

You would need to change this within the stream settings for whatever stream you are looking at. If you take a look at the following guide, you will see how do amend these to your liking