most likely its the lease time on your IP from DHCP on the router
Translation - your router will give your PC a IP address when it connects. There are only so many of these IP addresses so it sets a time (called a lease time) on this so if you turn your PC off it can reused the IP address again in the future. Often this is set to a 7 days but its changable to a difference number of days. IF your PC is still connected it will not take the ip away after this time runs out so dont worry it will reset the lease time over and over. This is only something that really matter in enviroments with over 200 devices attaching to the router (like in an office).
Hope that makes some sense.