What size pots are they in now? Usually for me I start off in Jiffy pucks to germinate. They I put them into 4" pots. Usually, depending on the plant growth, after 2 to 4 weeks I transplant to 5 gallon containers and that is where they stay. You can also just put them into 3 gallon pots a lot of people do that, I ususally keep the best plant for a mother grow one out and just keep the mom for cloning.
Their are signs to look for. If you notice that your plants are yellowing in the leaves and you can't figure out why it might be you are root bound. It will usually start in the bottom leaves and the soil will not hold water as long. You will usually notice some roots starting to come out of the holes in the bottom of your container. Just remember a pot that is too small will hinder growth and hurt your plant. A pot that is too big just takes up to much room. Becarefull with bigger pots that you don't over water. I had one plant that went from a 4" pot to a 5 gallon pail, I didn't need to water it for 3 weeks. After about three weeks the plant started to really shoot up quick. The roots really developed like crazy. the plant was super healthy, I still have her too. She made a great mom. You just have to really watch your babies, but a little love and they'll do good. If in doubt just transplant it, be careful when doing so and do it just a little before lights out so that they can recover from the transplant overnight. I also wet the soil with hydrothrive in the water to add some vitamin b12 to help with stress.