Formula for calculating the Length of Sales Cycle ??
Length of Sales Cycle = Average Time in Days between opportunity created date and closed date for all closed opportunity (“StageName = Closed Won” and “StageName = Closed Lost”)
There are a number of ways of doing this (least of all being using the Opportunity history report which you could probably get this from).
A nice simple way of doing this would also be to add a field to the opportunity that is set whenever the status of the opportunity becomes Closed Won or Closed Lost (this could be done via workflow). You could then use this field to cover off a number of scenarios where you want to display the average including on a report or in a page where you would simply have to do a small amount of math/use an aggregate query.