I had a similar question and from what I am seeing, the answer is 12 hours. If you do nothing, don't define a lease time, you are given a 12 hour lease.
show ip dhcp bind
lease 172.20.99.254 {
starts Mon Jul 21 04:44:36 2014
ends Mon Jul 21 16:44:36 2014
binding state active;
next binding state free;
hardware ethernet a4:eb:d3:**:**:**;
uid "\001\244\353\323\343\253\331";