Some servers have this but I don't know what it is or how to do it, but they have it set up where you get free VIP just by being active. But if you are inactive, the VIP is taken away. How can this be done? All I have right now is Server Rewards where you buy VIP with RP, which is sort of like that, but once you buy it, it lasts forever.
Plugin to give free VIP just for being active?
I know playerrankings or playerranks or whatever the plugin is called can give groups and stuff for playtime. ex. if you reach 100hours you become from PLAYER to MASTER PLAYER or whatever you set to be. Other way is with the RP store as you mentioned.
About the expire there's one trick you have to do. Not sure how other servers keep track of all shit like how active you are bla bla but for beginning you can give not full VIP group access just only timed VIP access via TimedPermissions plugin so they will buy or earn 4day or 6minutes or 21hours or whatever you decide as time and after time expires they will be without VIP again.