Here is a simple timer with a configurable time. The redstone block will be switch every [X] seconds.
So you can choose the speed of the clock/switch/timer by ajusting the quantity of items in hoppers!
1 item = 0,6 second
1 stacks = 42 seconds
2 stacks = 1 minute 24 seconds
3 stacks = 2 minutes 6 seconds
4 stacks = 2 minutes 48 seconds
5 stacks = 3 minutes 30 seconds
You can have a longer timer here : http://www.minecraft-schematics.com/schematic/1764/
Good Luck & Have RedStone !