You will be fine in May.
If you will go to
https://www.wunderground.com/q/zmw:32801.1.99999 you can find out your length of day. That is for Orlando, but just put in your zip and it will bring it up for your town. Just scroll down to the Astronomy section and check it out. A general rule of thumb is when your days are longer than 13 1/2 hours, it is safe. That is actual length of day, not length of visible light} But strains vary. The best way to learn is record when yours start to flower this year for each strain. Start a notebook, even if it's on RIU. Record start and finish dates so you can look back.
This may or may not help, but when you are figuring, remember that with most strains you have 3-4 weeks before the plants are sexually mature. They won't flower until they reach that point. But I wouldn't cut it too close if I was you. Too much down side for early flower.