few questions from a beginner

Have a few questions. Right now i have the lights on 18on/6off, how long do i have to keep the plant in vegatative state. I have read until the plant gets 16-20" then go into flowering stage (12 hours on/12 Hours off) Is this true?

How often do you guys water? I have a PH/water Tester so that should help. Any tips would be greatly appreciated. Thank you guys in advance!

You have some reading to do---> https://www.rollitup.org/newbie-central/176831-best-all-around-guide-ive.html

you would want your plant to veg until its atleast that sort of height. The longer you veg the bigger it gets with more nodes which results in more bud sites. Watering will depend on the humidity and size of the plant. you want to water it so all the soil is moist and then leave it until all the soil has been dry for 24 hours and repeat. The light timings you have stated are true. But going into flowring light cycle is completely upto you in terms of how much space you have for the plant and how long you're willing to wait too harvest.