Here is a simple timer with a configurable time. The redstone lamp will be light [X] seconds, every [2X] seconds. Then, you can take your output on any redstone lamp.
So you can choose the speed of the clock/switch/timer by ajusting the quantity of items in hoppers!
1 item = 1,2 second
1 stacks = 1 minute 24 seconds
2 stacks = 2 minutes 48 seconds
3 stacks = 4 minutes 12 seconds
4 stacks = 5 minutes 56 seconds
5 stacks = 7 minutes 20 seconds
You can have a smaller timer here: http://www.minecraft-schematics.com/schematic/1763/
Good Luck & Have RedStone !