|It all depends upon the server issuing the lease. It can last for hours or for years. With some ISPs (you don't only get dynamic addresses from ISPs) you will find that you almost always get the same address, even if you disconnect for a substantial time; with others you may get a new address every time you reconnect. |
There are no rules, or laws, about this. From a legal point of view I expect that the powers that be would prefer us all to have unique, fixed addresses (which will happen when IPV6 becomes widespread).