I’m using only 1 t5 right now but plan on getting an led for veg
You can run them up to 24 hours a day as long as it's not burning them. I run 18 on 6 off so they get a little break and just my theory but I think there's better root growth with short term darkness.
After researching lighting for a couple weeks. The system I use is 18-6 6 on 2 off. It's said to allow the plant stomata to close for a time so the plant isn't wasting light after a build up of photo products.
Look up Bruce Bugby on craptube. He really explains lighting if you like to know whats up. BTW you may have to do some base research to understand his info. If you want to grow good ladies you need to have some decent understanding of lighting, what makes it different (the types and spectrums are VERY important), and what is best for your goals.
Bruce is the man when it comes to light! I use 20 on and 4 off myself, I agree that they benefit from a small amount of rest.