For one account: TTV = timestamp(value event) − timestamp(start event). For a cohort: report the median of those durations, plus the share of accounts that never reached the value event.
The start event
Signup, contract signature, or kickoff call. All three are legitimate and all three give different answers. Self-serve products usually start at signup; sales-assisted implementations usually start at signature or kickoff. Pick one, declare it, and label every chart with it.
The value event
An observable, logged event that indicates value was delivered. It should be the same event you use for activation. If the two differ, you now have two definitions of value and neither team will believe the other's numbers.
The clock
Elapsed calendar time, including weekends and waiting periods. In-product time excludes exactly the delays you most want to see: the three days spent waiting for an administrator, or the week lost to an approval.
The summary statistic
Median, with percentiles if you want spread. The mean will describe none of your customers.