According to E. Trasolini Pool Construction Ltd. "Owners Manual and Maintenance Guide," you should wait at least 15 minutes before swimming in a pool which has been treated with muriatic acid.
Wait 15 Minutes
Wait Two Hours
According to Clean Pool and Spa, you should wait no less than two hours before swimming in a pool which has been treated with muriatic acid. According to the website, "If you pour muriatic acid in the pool and swim within 10 to 15 minutes you might get a 'hot spot' of acid that can burn you."
Muriatic acids can contain hydrochloric acid concentration levels between 28 and 35 percent. According to Scorecard.org, "Hydrochloric acid is highly corrosive and irritating to the eyes and respiratory tract. Acute inhalation exposure may cause coughing, hoarseness, inflammation, and ulceration of the respiratory tract." As with most pool treatment chemicals, it is advisable that you wait two hours after using this product before swimming.