Once a round trip is completed, then the rake gets primary maintenance anyways. So if a round trip is lesser than 3500km (ICF) or 4000km (LHB), then SM is not required.
If the round-trip is much lesser than 3500km, like for intercity trains, then the BPC is valid for 96 hours and trains can get PM anytime within that. So, intercities and other short distance trains, can get primary maintenance on every alternate day or once in three days, if the depot wants to avoid daily maintenance.
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The limit is followed quite strictly and most of the trains that exceed these limits will get a revision of their BPC, or are issued special round-trip BPC's to enable such journeys.