I'm no expert but we made a daylily bed 2 years ago and planted them approx. 2 feet apart and it looked a little sparse but filled in a lot the first year. This year, the 2nd year, the original plants are twice and a few three times their original size so I'm glad we allowed 2 feet. I planted Balsam (annual) all around and in the middle I have a Yucca plant and 4 Echinacea. I've posted pics below to show the dramatic change in plant sizes in just 2 seasons:
16 Jun 2014
25 July 2014
14 July 2015
18 July 2015
Best of luck with your garden. We love ours.