Generally speaking; most brands state an expiry date of one year from manufacture on their sealed powdered products but in mixed powder form many ‘lents will last well over two years.
Once opened powders should be ok for a few days to a week, or longer if properly sealed between use.
Things change once you add water. If you keep it sealed, and cold, it should last between 24 to 48 hours.
Don’t leave it on the kitchen counter or anywhere warm for more than a few hours because what’s great for us is also great for fermenting bacteria.
You’ll know if this is happening, as the bacteria produce co2 you’ll get a loud pop when you unseal your shaker that’s accompanied by a rancid sourness that even the most hungry future fooder will want to avoid.