how many days into flower til plant starts stretch...

Stretching depending on strain, but mainly first 3 weeks after flip to 12/12. Some varieties grow with colas up to 6th week of flower. Switching to bigger pot is good , but 5 to 7 is too small jump, you make more harm, try to double. It saves from many mistakes made in/with soil. Keep plant dry before transplanting, roll down old pot like sock or take out and use good soil. Water for first two weeks with just ph adjusted. You can use Micohrryzae for roots. Good technique is to "prune" roots abit. People cut small crosses through bottom off root mass. Google it. Works well.