You don't say where you live, so it's difficult to put a date on it, but just as the plant is to begin its summer push is best.

Because plants in bonsai soil often don't show the effects of soil collapse like plants in organic mixes, they often don't exhibit much in the way of signs that they are struggling. Foliage might look perfectly healthy on a declining plant for several years. If your tree is not rising in the pot and showing roots crawling over the surface and visible at the drain hole, all indicators, reduced branch extension is still another telltale and key signal that a repot is in order for most trees.

Your tree may pout for a couple of weeks after the repot, but its development, by summer's end, will have surpassed that which would have occurred had you not repotted. Tight roots slow development.

Al

Pick the tree up by the trunk with one hand and with the other gently tap on the rim of the pot downward to ease the plant out of its pot. If you see that the roots are circling the pot and matted together, repot when you first detect new growth in spring.