How long can you keep normal saline once its' been opened?

What is it being used for?

If it's normal saline for irrigation it's designed for single use so any unused solution should be thrown away.

If it's saline for contact lenses chances are it contains preservative and will last for 60 days after opening - check the label on the bottle.

If it's normal saline for injection with a bactericide it will last for 30 days - again check the label.

Discard unused portion after 30 days rings a bell.