We'd recommend to first ask the PT how much time they would need to decide.
They might say something like a couple of days, etc.
Otherwise, the customary amount is no more than one week.
(The offer letter would typically have an expiration date written in.)
We recommend to continue to engage / interview additional candidates even with an offer extended. PTs often decline job offers and you'll want to keep the process moving with additional candidates to potentially hire.
Also, if you've already interviewed 2nd choice candidates, we recommend to check-in with them every few days, just to say that you're still interviewing and plan to have the process wrapped up soon. The "check-ins" can take place via phone, email or text.
(This keeps the 2nd choice candidates "warm" in case they become a 1st choice candidate at a later point.)
We do not recommend letting 2nd choice candidates know that an offer has already been extended. No one likes to feel like a "Plan B" and PTs will often lose interest in a position if they know that an offer has been extended to another candidate.