We have only just recently starting using Kanban, so we have little or no predictability data about how long epics and stories take to move through our system.
How do we give the business an idea of how long epics and stories will take to exit the system?
Ideally you wait until a number of stories and epics have passed through your Kanban board and then use that data to help estimate future cycle times.
If you absolutely need to provide estimates from day one then you can use the following approach:
This is similar to traditional development estimating and as such is vulnerable to over-optimism, pressure to size estimates based on business needs, etc. There is also an argument that this kind of estimating is a form of waste as it takes the team time to do but provides only limited value.
I would recommend that if you go with this approach that you drop it as soon as you have a reasonable amount of cycle time data available (say after a few weeks or a month).
Actually you can use little's law to do this, assuming you have at least some (at least 2 I guess) items leaving the system (i.e. they were started a long time ago before you started using Kanban) and you have a throughput. (It is not required to have any items traveling all though the system - I did this at a company that had lead times of 300 days plus, so weren't going to wait that long before predicting lead times)
Expected Average Lead Time = Current WIP / Throughput
That sort of system is all about trends. You can't provide data you don't have, so you need to start building it up by doing the work.
