At first, let me shortly explain how Block Erupters (generally known as icarus miners) work.

1. Computer running mining software (like cgminer) sends a task to the erupter.
2. The erupter tries all 2^32 possible nonces.
3. If the erupter founds a valid nonce, it sends it back to the computer. But if no valid nonce is found (task cannot be done), nothing is sent back to the computer.

So the computer must wait some time for some nonce or after some time consider the task as finished without the result. This time is in case of Block Erupter approximately 12.78 seconds (2^32 hashes / 336MH/s). Unfortunatelly cgminer (according to git sources - https://github.com/ckolivas/cgminer/blob/master/driver-icarus.c#L81) waits only for 10s. So it misses some valid nonces. But this is another story.

Now about the meaning of led states:

• Led is OFF - erupter is currently working
• Led is ON - erupter is waiting for a new task
• Short flash - erupter have just found a nonce. It is possible that Erupter finds multiple nonces for one request. I am not sure if it will flash multiple times in this case.
• Long flash - is actually short time of ON (idle) phase between two OFF (working) phases. It means that erupter finished work but computer have not sent next request yet. This is probably due to some error or just because computer has no task for erupter ready yet.

Normally long flash should happen after every request. But due to the 10s issue erupter gets a new task before it finishes the previous one.