Need more info here mate. Pot size is preferably given in gallons or liters or at least give us the dimensions in ..x..x.. so we can work out the volume. Diameter doesn't help unless you are using round pots... 6 weeks old since when? Since sprouted, seedling, vegetation started? Are you on soil, what lights are you using and what strain is it... Also your growspace, how big you want or can afford your plants to get? Are you still in vegetation or you are already flowering? Basically it mainly depends on how big you will allow them to get hence how long you will leave them in vegetation. You can grow a plant fully in a 1 gallon pot and you can do the same on a 20 gallon pot. There are many parameters so give us more info!
That said if you just want a simple rule of thumb, then when you see roots coming out of the drain holes, growth slows down, plant needs to be watered every other day, then that might as well be signs of rootbound so you can then transplant to bigger containers.
