Bukkit

With this plugin, you can promote player after a certain time. To use this, you have to enable it in the config:

Time:
  Enabled: false

After the player has been on the server for the time, he gets promoted. All your groups except your default group have to have the permission 'MasterPromote.member'

Time:
  Enabled: true  //enables the time promotion
  Group: Member
  Time: 10  //time in seconds
  CountOffline: false //if you set this to true, the time would count even if a player is not on the server.

With this configuration players would get promotet to Member after 10 seconds playtime.