Dormancy is based on inactivity around your core action for a set period of time.
The time interval will be specific to the natural frequency of the problem your product helps people solve.
Let’s say I run a publishing app that caters to people who publish content on a weekly basis. If a user stops publishing content for a month or so, they’ve lapsed into the danger zone.
The exact period of inactivity that qualifies for dormancy also depends on your product. If you draw the line too soon then you’re just spamming people with reminder notifications while they’re on holiday. Leave it too late and they’ll completely forget about you.
To narrow in on the optimal timeframe to reach out to people you can work out what percentage of people return based on how long they’ve been dormant.
Maybe 90% of people who go dormant for a week return to the app. On the other hand, only 12% of people who go dormant for 12 weeks ever return. What you are looking for are sharp jumps. If 70% return after 3 weeks but only 40% after 4 weeks then that’s fertile territory for an outreach campaign.