Day 55, this is a photo period. It is really up to you based on the size of your tent, the amount of time you have for training. Plants usually stretch about 30-40% when they go into flower, some more some less. You have to be the judge of the space. When you are ready for it to flower, switch to a 12/12 light schedule and it'll show preflowers/sex in a week or two, with buds coming in after that.