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how many weeks till you change your pot size to very large ????

In an attempt to keep the "hassle factor" to a minimum I would advise going to your largest pot on your next transplant. This does a couple of things. First, it negates any chances of transplant shock. Secondly it gives the roots plenty of room to expand, both downward and outward without any chance of becoming rootbound. Plus, it gives you more room if you plan on training your plants to maximize yield. I'm going to be starting a new grow in a few weeks and I'm going straight to the largest pots my garden will accommodate.
 
I would transplant as needed based on the size of my plants and would not go directly to the largest pot possible, this allows the root mass to better fill each smaller pot before going into the next size up pot, a lot of new growers mistake transplant shock(a slowing in growth above the soil) for redirected energy going to build or expand the root structure. While transplant shock does happen it is not common in my experience, with a healthy plant.