Formula
Average time from invite sent to invitee's first invite sent
How to measure
Timestamp every invite sent and received per user, then measure the average elapsed time until the invitee sends their own first invite.
Why track it
Shorter cycle time compounds growth faster than a longer cycle at the same K-factor, since more viral generations complete per unit time.
Example
A collaboration tool shortens its viral cycle time from 9 days to 3 days by prompting invites immediately after first use instead of after a week.