You have come this far, wait until you are sure there are no more hard frosts. Al

6 inches! Mine are still only half that (but look sturdy). Delphs are usually bone hardy so you could harden them off over the next week or so, then get them in the ground. But (there's always one of those), the worst thing about planting them out is not weather but exposure to the slug and snail hordes, all of which will crawl over any amount of coffee grounds, eggshells, brambles and dried holly leaves, to feast on gourmet delphinium.
When did you sow yours, Clairey? I did an October sowing and a January sowing - the early ones had to be chivvied over the winter and are actually weedier than the ones sown in Jan.