General Guidelines. Your veterinarian should tell you how long your cat needs to fast before surgery, but there are some general guidelines. For cats younger than 4 months old, withhold food should be withheld starting about four hours before the spay surgery. Cats older than 4 months old should not eat overnight before surgery.Some veterinarians will recommend removing food at midnight, others recommend 9 p.m., and others will advise you to restrict food whenever you go to bed. Many times the difference in times depends on the time surgery is scheduled.ats older than 4 months old should not eat overnight before surgery. Some veterinarians will recommend removing food at midnight, others recommend 9 p.m., and others will advise you to restrict food whenever you go to bed.