Is there a certain age of the clip that makes the sample free for use? How about the length of the clip? Basically, is there any sample I can use for free in instrumental production? Thanks
Fair use is about use for study or educational purposes or for making a copy where obtaining the item at a fair commercial price in a reasonable time is not possible.
What you describe does not fit either of the clauses.
If you were producing something for educational assessment it would be fair use.
If you were to subsequently release it you need a licence to do so.
Length of the clip is immaterial to fair use, but is material to the licence fee that can be reasonably charged.