It's in the current Sheperding book, which you should be able to find online.
The Sheperding book doesn't mention a time period, says a "length of time" or some such.
There is a letter to elders that more or less stipulates as a clarification that that is 3 years.
The SOLE KEY is "Jehovah's blessing" ; I.e. lots of service, lots of meaningful comments - the same key of whether or not to DF Joe Publisher.