No great rule of thumb really, each one likes to be a certain length, and even that varies batch to batch.
We used to get a batch of these around holiday time for several people in the office.
Murray's Cheese - La Tur
Real good, you kinda gauge when it's ready by poking it, have to get a few of them until you find your point where you think it's the best.
And agree on the above Humboldt Fog recommendation, that stuff is GOOD!