Q.When To Add Lime To Cabbages
Do you add lime when they are ready for planting in the ground or seedlings in a pot?

Certified GKH Gardening Expert
You will want add it when a soil test confirms that it is lacking. This will not have a set time period.
Ultimately, your lettuce will tell you if you need to add it by the tips of the new leaves turning brown, but it is best to test the soil to make sure that all nutrient levels are correct prior to planting.
Once a test confirms calcium deficiency, it can be used in the soil, or sprayed on the plant in foliar spray form at any time.
These articles will help you to test your soil:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm