When do I switch my plants from 18-6 to 12-12. Can post a pic if needed
When you want to flip is up to you. 
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It takes plants about 4-5 weeks to mature on 18 hour schedule, so flipping before that doesn’t make a lot of sense for most people. From there, you just want to make sure plant is large enough to reach your goal without being so big that it over grows your space.
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To add to what’s already been said, if you plan on pruning, you want to accomplish that before the light change. It’s also understood that you want to make sure the plant is in good health and free of pests before initiating flower; issues are often easier to remedy in vegetative growth.