question or two in one. When should I start pruni...

Our friend @DudeGrowsWeed is right, even if topping is much appreciated by many for its simplicity and its 'almost-immediate-visible-results', I'm now thinking that non-destructive methods are more efficients for autoflos. Auto's lifetime is too short to allow plants to be topped multiple times. I suggest you to take a look at the other methods you could use : FIMing, PonyTailing, Pinching, Bending ... These methods won't affect the plant like a scissor cut would, so recovering time will be really fast and your plant will develop numerous strong branches without getting hurt. You can start working with these methods from week 3 / node 3 or 4. I'll be around, good luck with these babies!

I’m not opposed to topping autos, I’ve done it myself with good results. But I’ve become a bigger fan of FIM pinching rather than topping. I wait until there are at least 4 nodes, then give a good pinch on the new growth at the top - I don’t remove any plant material at all, just pinch it together firmly. In my experience this causes a lot less stress on the plant and still promotes more colas and outward growth. With defoliation I wait until the plant gets very bushy first, then remove the fan leaves that are blocking light penetration. I try to leave as many leaves on as possible while creating better light penetration. I usually wind up defoliating twice, once around the end of veg, then again a few weeks into flower. The first set of leaves I leave on until the start yellowing. Hope this helps.