Disclaimer: I've not ever grown cannabis... ever.
All the advice your getting here is right from the research I've done, but your missing the point ....
You CAN veg them as long as you want .... years if you wish for them to be mothers for clones ....
You could probably stick them into flower right now. You'll have fewer bud sites and the plants will be short.
It all depends on what you want ....
The longer you wait to flower the more nodes you'll have which means more potential bud sites ...
The less time you wait you'll get fewer nodes, fewer buds, but shorter plants.
It all depends in your space. You said 'small grow room'
What's 'small?' Computer case, closet, or bedroom?
Between 3 - 4 weeks into veg you'll notice 'pre-flowers' these signify two things, first that they're ready to flower (maybe not ideal but they CAN be flowered) and second, with a good eye and some research you'll be able to tell the sex of the plant.
If you're running low on bud and need something to smoke, take some clones, start rooting them, and put the plant in flower. It you want massive cola's and many many bud sites wait.
Once in Flower the plant will anywhere from double to triple in size. So 2 foot tall plant in veg can and likely will reach up to 6 feet tall. This won't work in a 'case' grow, might in a closet grow, and will in a room or outdoor grow. If you have room and want more buds then bend or SCROG your plants and grow the horizontal and you have low plants with several big colas point up. So the answer to your question is as soon or as long as you want as long as they've reached pre-flower state.
You have to make your own call and do some math. If your room is 9' high you can probably get away with flowering at 2.5 - 3 feet, if it's 6' high you want to flower as soon as you possibly can. The less light you have the sooner you want to flower as lower vegetation won't get light near the bottom so you won't get good dense buds down there.
My grow room that is under construction is 7' 6" tall. I plan to flower at 1.5 feet at the most because my HID is going to take up a foot of that height reducing my usable space to 5'6" after the size of the reflector and 1' to keep from burning the plants. So at MOST my plants should never reach above 5', if they do I'll have to super crop, bend, scrog or something to keep them from growing into the light and out the cieling. I fully intend to top them to keep them as short as possible with as many top cola's as possible since they won't be that tall.
So what is small, what do you want out of the plants, and how much light do you have? Here's a simple formula.
Short room, low light = flower ASAP
Tall Room, lots of light (HID) = decide on what you want out of the plant.
Lots and lots of buds = veg as long as possible
Good top cola's and a few small popcorn buds = veg until preflower and then flower.
Of course the best combo is LOTS of room, LOTS of light, and plenty of time. If you're running short you can do a small SOG and flower at 1 - 1.5 feet but still get lots of bud by adding new (clone) plants every other week or every week.
Your question isn't 'easy' to answer because there are too many variables. Simply put do what you want as long as they've preflowered. If they haven't wait, if they have and you need bud, flower now. If you want more bud and have the space and light wait anywhere from 2 - 4 months after seedling state to flower.
-RT76