Wait at least 15 minutes, but check the label.

Most algaecides are safe to swim in after 15 minutes once the pump has circulated the chemical. But the real answer depends on the specific product you used — some require an hour, others are fine immediately.

The main risk isn’t the algaecide itself (most are mild), but that you might have also shocked the pool at the same time. If you added algaecide and shock together, wait until chlorine levels drop below 5 ppm — typically 8–24 hours.

Your pool will still be there in 20 minutes. Better to wait a little than to itch your whole swim.