There isn't a good definition. At Dropbox, we try to answer the question: what percentage of signups 56 days ago were active 1-28 days ago? For larger time peri

There isn't a good definition. At Dropbox, we try to answer the question: what percentage of signups 56 days ago were active 1-28 days ago? For larger time periods, just look at the sum of each day over the window (sum-of-retained/sum-of-signups. 28/56 day windows are useful to keep the same number of weekends. This reports the most recent signups for which there is good data, but is lagging. You could look at another action that causes people to be retained that happens earlier in the funnel to run an actionable test in a reasonable period of time. Looking at cohort analysis historically will get you a good understanding of the percentage of users that are active after N time periods.