How long will diluted shampoo stay fresh?
Shampoo should always be used freshly diluted. Water has bacteria in it that can make pre-mixed shampoo or conditioner spoil over time if left too long. A general rule is to use pre-diluted shampoo within 24 hours in summer, or 48 hours at cooler temperatures (as bacteria grows faster when it's warmer).
One easy approach is to mix the shampoo and conditioner needed for the day, in the morning, then empty and clean out containers at the end of every day. Make it a routine to avoid accidentally using spoiled product.