It does vary by school, I think it's generally within a 5-7ish year range for most I've seen...but some might be less or more than that. You may also want to look into the schools' policies because I've seen people on here mention that they were able to get exemption from retaking certain old prereqs (like general chem, or bio) by demonstrating a "continuing/advanced involvement" in the subject area (although what things might qualify as that, or whether the school will even grant such exemptions would also depend, I guess). I thiiiink LSU's expiration is at 6yrs and so I retook bio 1 last semester because the original was a bad grade and 5 years old.
