How long does it take to activate gift card?

Usually 24 hours. If it doesn't activate, the store forgot to do so. If that happen, just take the card back to teh store and ask them to either activate it or replace it. 😁

Welcome to the Apple Community.

When you redeem a gift card the credit should be available straight away, there is no forgetting, it's not done by humans. Do you get any error messages.