It depends. I like it well fermented so I taste it first. Some kimchi is already done straight out of the jar (especially if the jar was stored even briefly at room temperature) and doesn’t need aging. The unfermented kimchi is crunchy and bitter, the fermented one is soggy and sour. To age, I take the jar out, loosen the lid and let it sit at room temperature for ~6-16 hours. Once the fermentation gets going, the liquid will bubble over, so place the jar on a plate (or relieve the gas with fork once in a while). Then keep cold and enjoy. It comes down to your personal preference though, some people like kimchi not well fermented.