The right time for surgery when you have an isthmic spondylolisthesis at L5-S1 is when symptoms prevent you from doing activities you desire to do or motor weakness is noted. Since the L5-S1 foramen is compressed and contains the L5 nerve root, you need to know what muscles could be affected that you can test.
When the tibialis anterior (which brings the foot up) is weak, you can’t walk on your heel on that side. Simple “duck walking” (walking on the heels without your toes touching the ground) is a good test. If the toes eventually start to touch the ground, you have weakness. Also the gluteus medius muscle is an L5 muscle. In something called a “Trendelenberg gait”, with weakness, you cant stand a one legged stance on the opposite side of the weakness.