That would depend on how big of a bonsai you want to make it. The rule of thumb is to let it grow to a width 1/6th the height before you do your chop; you can't expect a tree to put thickness on at the base until everything above it has caught up, which rather ruins the tapering effect.

I would let it grow, and wouldn't be reluctant about moving it into a bigger pot to facilitate that. If you must keep it in something small, get a shallow mesh basket or collander to start training the roots.

Geography (where you live) has an impact on when pruning and repotting should occur, and the degree of impact increases with how many degrees of latitude separate you from the equator.

I chop at least 90* of the trees I grow to enhance the trunk's taper. There is nothing stopping you from developing your tree while you leave the existing trunk attached for several more years as a sacrificial part of the tree. As Steve partially explained, the part of the trunk distal to the mark you overlaid on the image is an asset to the future composition, and can remain as such for several more years. I urge you to learn how to formulate a sacrificial branch PLAN for most of your trees. Example:

A larch before any work was done. ^^^Here ^^^, I have established the trunk which emerges from the soil angling slightly to the right, the branch wired with heavy wire that moves left is the trunk line. The main trunk above the branch (with wire) on the right, is sacrificial. It will never be a part of the composition. You can see a second order branch (first order branches grow from the trunk. Second order branches grow from first order; third order from second order, .....) coming off the first order branch with heavy wire and moving back to the right. You can see 2-3 small branches growing to the left where the trunk changes direction from left to right - where the wire gets smaller. One of those branches will be kept. as a branch. The heavy branch beyond those 2 branches is sacrificial. A couple of years after the main trunk is chopped, that branch will be removed immediately distal to whichever of the 2 small branches I keep from where the trunk changes direction back to the right. All of the branches below the wired branch moving right are also sacrifice branches being used to fatten the trunk. I can choose to remove them or jin them at any time in the future.

Also, as you consider what style you wish your should be, make sure you orient the tree so the major sacrificial wounds will be on the back side of the tree, where they won't be seen when the tree is viewed from the best angle (the front). . This should always be a part of developing young material - not always possible with trees that have established trunk lines you don't think you can/want to change.

At this point, the future tree is all just above the soil line. To allow sun to those branches intended to be a part of the future composition, branches immediately above them were removed. With a little imagination, you can use your mind's eye to picture your nicely tapered tree 5-10 years from now, and set out on a plan to bring it to fruition. Good luck. Keep in mind that that design is a penultimate goal, secondary to maintaining your trees in a high state of vitality.

Al