When to stop veggy and start flowering for my whitewidow baby-

Hey guys, new to this site- posted something a while ago but didnt get much replies- tryin this again!

I have been growing my greenhouse whitewidow in its vegetation phase for about a month and 10 days- its a closet grow- so im wondering if if anyone knows how much it may stretch- its supposed to flower for 8 weeks and i was thinking about starting flowering on the Dec 1st which will put the veggy at about a month and a half in. Shes a very happy plant! please reply with any feedback thanks!

ATTACHED A PIC- GROWING IT UNDER A UFO

your plant will double, or if it's a sativa then it might even triple it's size when you flower it. So it's really up to you how much space you have.

thanks- i just dont want the size to decrease the yield too much..

And i was also wondering if i should change pots?? i have it in fox farm soil in a reasonable/medium size pot.. but how do i know when or if i need to switch it in fresh soil? would it be better for flowering or how exactly would it effect the plant if i chose to keep it in same pot. Or does it matter?! haha