When it proportionally looks right (aesthics) in comparison to the size of the tree.

Here's a brick circle I put around an Acer japonicum 'Acontifolium' and another around a Picea engelmannii 'Bush's Lace'.

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One of the advantages of tree rings is that it helps keep other plants away from the tree. If you are growing your tree in a circle surrounded by lawn grass like in the spruce photo above, then it is easier to keep the lawn out, which helps keep mowers ands string trimmers away from the tree.

BTW that is a very nice looking 'Aconitifolium'! I find them to be extremely tolerant of fairly windy locations.