If it's before they sign the contract, then they are simply sent home.
If it happens during their service, say months later, then there are several things that can happen. There are variables. They can be sent to Dry Dock for rehab, they can challenge the test and be retested, they can be discharged, or they can get a combination of those things. It depends on the timing, the circumstances, the member's record, etc.
I don't know the proportion.
TCS might have more info because he was a DAPA officer. I was merely the person who observed other women pee into cups, including women officers.
