Thanks for posting your question. I'll answer your question and then a bit more to give you a better understanding of the time frame from adult to cocoon to adult again.
Worms are not adults until you see the clitellum (the thick band or ring in the near center)on the worm.
Although worms are hermaphrodites it still takes two to "tango". Once the sperm is deposited in the clitellum it slips off into the soil as a cocoon.
The cocoon can have 1-4 worms inside. This can take 2-3 weeks to hatch and this really depends on temperatures though.
Higher temps will stimulate faster growth rates inside the cocoon while lower temps can stimulate the mating cycle.
From the time the baby worm hatches from the cocoon to the time it has a fully grown clitellum will be around 4-6 weeks.
So the whole process from cocoon to cocoon or from adult to adult cam take about about 2 months (respectively).