Begin with a fairly large capacitor, like maybe 100uF. RC is about 1/2 second with 100uF and about 5k ohms. RC is about 25 seconds with 100uF and 250k ohms.
make squire wave of 1/2 sec. divide it by 50 using 4060 as audioguru told, every reset will give you 25 sec delay and you can get 1/2 sec by another out put.