This is old but unanswered.
To produce a long off and a short on you need two clocks. I used 2 hopper clocks and a pulser to do it.
A hopper clock consists of two hoppers outputting or pointed at each other. One of the two hoppers has items in it. Comparators read those hoppers and move a redstone block back and forth to cause the items inside one hopper to transfer to the other. The amount of time it takes those items to transfer determines the delay so the more you have, the longer the delay.
Important: In the long delay clock, one of the pistons is a sticky piston and one is a regular. In the short delay clock they are both regular.
The pulser is simply a block with a sticky piston under it. It takes a long output signal from first clock and outputs a small length pulse.
Add items to one hopper in long delay clock to control off length add a stack of 24 items to short delay clock.
You can extend the length of the on pulse using another comparator and a chest. Fill one of the short delay hoppers with stacks of cobble and play with filling the chest to different levels to control on length.
Note: On length has to be shorter then off length. It will run oddly otherwise. Though if done correctly, longer off delays can be achieved.
To reverse operation and have long on with short off move the redstone dust at the output to be powered from the redstone block's other postion.