Automatically changing user Roles based on length of membership

I would like to show additional content to members based on length of membership. Basically the permissions are increased as the member goes from "Role A" to "Role B". I would like for the role to be automatically assigned to the member based on the length of membership (how long the member has been a member).

Permissions are assigned to "Rule A" for example, that are less than / different than "Role B" permissions.
As the member reaches a length of membership that qualifies for "Role B", they are automatically assigned to "Role B" and thus have additional / new permissions that are associated with "Role B"

Any insight into how to create this would be appreciated.

Thanks in advance,

Scott