How much light is needed to develop roots on a clone?

Not much. It can't take in enough nutes to do anything but cannibalize itself. A minimal amount of paling is fine but you don't want them to eat themselves too much before they get some roots. Use Bti to avoid gnats and more, if available in your area. "Mosquito bits" and dunks are the brand i know of. But anything with Bacillus thurgenisis (spelled wrong but close enough) will work on larvea and 100% safe for you or pets. I use bti pre-emptively. You never know when a bale of substrate will bring in a bunch of pests from the store. Give them weak light and low VPD until they exhibit signs of growth, and improved turgidity (they stand up straighter with greater sustained water pressure inside the plant) as far as how much light.. just make sure they don't pale too much or fall over trying to avoid the light. High humidity slows transpiration (low vpd). This will reduce stress on the plant and allow it to form roots more easily. Unless you live in a very humid region, a humidity dome is very useful. They key is not giving them uninterrupted darkness for 10+ hours (flower cycle / chance for revege etc), which is easy enough to avoid. Can they root simply plopping them in some substrate and leaving by the window sill? yes, but success will be lower. Always take more cuttings than you need. Sometimes you'll have a 100% success rate, others you'll encounter a stubborn genotype or maybe it's not as healthy as normal etc... Lots of things can reduce success rate. if you take 4-5 cutting you could just leave them on a counter and at least 1 would survive, lol.