There is already a limit to the time that a substitute can be played, 50% of the time on a world. Unfortunately the system isn't perfect.
The ability to control other peoples accounts via substitution is more important now than ever because of the covid-19 pandemic. 4 weeks may not be long enough if someone is self-isolating or shielding and lives in a house with only one PC. 12 weeks is the time the UK have told people with certain problems to avoid contact with others. Sharing a PC certainly wouldn't be maintaining social-distancing.
