"Duration" equals to plan's lifespan. On the 20th day your building will expire and you get back to level 1.
Wait, if it expires on the 20th then doesn't that mean I was only able to use it for 19 days? Shouldn't it expire on the 21st if it has a duration of 20 days? I'm planning to purchase level 3 or 4 in a couple of days and would like to clear this up first. Thanks!