There is a Pandas dataframe: userId movieId rating timestamp 0 1 31 2.5 1260759144 1 1 1029 3.0 1260759179 2 1 1061 3.0 1260759182 3 1 1129 2.0 1260759185 4 1 1172 4.0 1260759205 Add an "av_ltv" column to the dataframe, in which you calculate the average lifetime of the users. Lifetime refers to the difference between the maximum and minimum values of the timestamp column for a given userId value.

df.groupby('userId')['timestamp'].agg(['max', 'min'])
df['av_ltv'] = df.groupby('userId')['timestamp'].agg(['max', 'min'])['max'] - df.groupby('userId')['timestamp'].agg(['max', 'min'])['min']