To be honest, Alcor should publish a cryopreservation minimum chart for each year through 2110 or even beyond that.
10,000 AD 10 Trillion
To be safe, and economical, buy the insurance as early as you can in life that corresponds with your "deanimation expectency". Mine would be 2070 so I'd have to have purchased 200K.
One of Platt's articles has a chart like this... link it here if you can find it.