How much dirt can I leave in my earth boxes from last years garden?

I leave about half.

Potting soil?

Hi Ma Turtle. I'm Cindy. I think it would be OK to leave the bottom third but mix it well with new soil. Also put sand in the mix. It will help with water retention. Good luck Ma and Happy Holidays.

Some say dump all of it but I’ve had success reducing by half adding new soil

I usually replace about 1/3 every year, mix well; but use a good quality bagged soil... most window boxes are for annuals, that you will be feeding on a regular watering schedule anyway ...

I never remove the dirt. Remove all dead materials and left over roots. Heavily amend the soil with a good organic fertilizer recommend by your local nursery.

I usually leave half and mix in a fresh batch of mushroom compost with fertilizer the next year.