Picked up a 2015 Azera with 8,200 miles. No active XM subscription. I called in to Listener Care and told them I was renting a car and was a prospective customer. They activated a free 30-day trial within 4 minutes. No need for an account, phone number, address, etc.
I talked to a friend of mine who's relatively high up at corporate GM. He said that the manufacturers and XM agreed on shorter trial periods. Most of GM vehicles are 30-days, regardless of miles. Some are even shorter.