A/C works fine blowing cold air for a little while and then blows out warm air?

You are correct that the most likely cause of this is freon is low so let's start with this.

We need to check the pressures and find out what is there. Once we know the high and low side pressure then that will tell us what the issue could be.

Here is a guide that will help with this:

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

If you do need more freon then here is a guide on how to add some.

https://www.2carpros.com/articles/air-conditioner-how-to-add-freon

I am attaching a guide that will help with determining what the issue is based on the pressures. So, let's run through all this info and let us know what you find with this.

Thanks