Here's a video where I compare the sustain on four different harmoniums.
A good harmonium ought to sustain a 2-note chord for 4-7 seconds. Any less than 4 seconds and you'll have to pump constantly, which is a distraction.
How does your harmonium stack up? Here's a four step "sustain test" that will give you useful info on the state of your instrument.
As a benchmark, a Bhava Mini harmonium will have a 4-7 second sustain. You can expect a slightly lower number for a smaller instrument (like a Bhava Lite), and a slightly higher number for a larger instrument (like a Bhava Classic).