Is there a legally required time limit to register a land deed at the courthouse? And could a deed become invalid if not registered within a certain amount of time from being signed?

Generally not, but it might vary from county to county. Why do you ask?

As noted normally there is no time frame, BUT, a dishonest person, would still on court records show the property in their name, so they could actually re-sale it again ( illegally) but if they recorded their sale, then you would not be able to record yours, without a court trial .

You should record it within a few days of the sale.

Also the tax records will be going to the other person, so this would be an issue.