IANAV, but I did work as a tech for a summer in college...
Fixing a young male cat or dog is a relatively short, non-invasive procedure. The incisions are small enough that stitches are not necessary, surgical glue is sufficient, even with small dogs. If the animal has an undescended teste, the surgery is more complicated and invasive, you have to go into the abdomen and get it out. The larger the animal, the longer it takes, but neuters are pretty quick procedures. You shave the testicle, make an incision, pop the teste out, clamp it/knot, cut it, cauterize if there is any bleeding (rare), repeat for #2, close and go. It really depends on the surgeon as to how much time it takes. My sister was fresh out of vet school the summer I worked for her practice and she took longer than the other Dr in the practice who lined them up and did 5 in the time it took my sister to do two. This
(Warning: SURGICAL PROCEDURE, video of kittens being neutered
) shows an experienced surgeon doing and narrating 4 neuters in a matter of a few minutes.
Spays are full-on abdominal surgeries, regardless of the age of the animal. Spays on adult females can be complicated if the animal is in heat or pregnant. This
(Warning:Photos of surgical procedure
) shows and explains the steps in a canine spay. A normal, uncomplicated spay on a healthy young animal is usually about 20 minutes or so with a experienced surgeon.