To show how quickly our plug plants grow to fill the space I planted up 10 of each of our stock plants and will take photos to show how they progress. I recommend that our plugs are planted roughly 40cm apart (less for some of the smaller plants) and in 18 months the emerging clumps should halve the bare ground between each plant and when in full glory will be touching. This is what you want - solid planting but not so crowded that they begin to fight each other for space. This also keeps most of the weeds under control. I cannot bring myself to leave a trial bed completely unweeded but I find I can get away with just removing the worst weeds in spring and then just pull out any horrors when passing - our plants are tough enough to win through the rest.
So, if the plugs look a little sparse when you first put them in then I suggest adding some annuals (seed is the easiest) just to cover some of the space. I recommend, phacelia or borage as they will maximise the nectar and pollen supply until the perennials get established. Calendula is also useful as it will fill space quickly and flower for 4 months.
This is how the look in early September - only 3 months later
Result: the will double or triple in size within 3 months from spring to autum or from autumn to the following summer.