Camphor Southernwood Dividing and Longevity

My question deals with my camphor southernwood. I have had this beautiful plant for at least 15 to 20 years, my husband split it once several years ago by using a shovel. The plant survived and now I have two beautiful plants. They are very large however and I want to split them again so I can make a hedge-type appearance across the front of my house. I just want to know when it should be done and how it is done properly so I don't lose any of the plant. Also, do you happen to know what the life span of these plants are? I just bought another from Richters to add to the other plants I have because I'm just not sure if someday they'll just stop growing.

Early spring is the best time to divide the plant, just as it is starting into growth.
Barring accidents, neglect, disease or insect infestation, your plant should easily outlive you! Shrubs and trees live far longer than many herbaceous perennials. Renewing the plant by dividing it prolongs its life even more. We have propagated new plants from our stock plant for 25 years or more and don't know how old the plant it came from was. So do not worry.

Could you tell me what zone I live in, it's central Pennsylvania (Williamsport), USA.

Central Pennsylvania lies between zone 5 and 6.