import pandas as pd
JSON = [{'part':1, 'time':"2022-08-15 8:17:12"},
{'part':1, 'time':"2022-08-15 8:18:20"},
{'part':2, 'time':"2022-08-15 8:21:14"},
{'part':3, 'time':"2022-08-15 9:02:03"}]
mn = pd.to_datetime(pd.DataFrame(JSON)['time']).diff().mean()
print(mn)
'''
run:
0 days 00:14:57
'''