A request is not a booking. When you ask for a time a studio does not have open, or a studio that reviews every booking by hand, the request sits as pending until they answer. Nothing is held for you while it does.
Most studios answer within a day, but they are answering between clients — a professional with a full column does not see a request until they are between appointments, and a studio closed on Mondays will not see a Sunday request until Tuesday. Their opening hours are on their page and are a fair guide to when to expect a reply.
If two working days pass with no answer, send one message on the thread, and start looking elsewhere at the same time. A studio that is slow to answer a request is often simply full.
You can have more than one request open at once, and you can cancel a pending request yourself at any time before it is accepted. If you find something else first, cancel the others rather than leaving them hanging — an accepted request you no longer want becomes a real appointment, with real cancellation terms.