I've kept vegging plants in solos for months.Once the cup is fully rooted,they will require frequent waterings.To answer your thread question,as long as you can keep them healthy.
What size pots are you going to flower in?
What is your desired height of plant(after stretch)?
Have you run the strain/s before?
If you are going yo veg for 16 more days,up-potting may be best at this point.It will give the roots a chance to get thru the new medium and get settled good before you flip to flower.
You will know when they are unhappy and want new shoes.