Generally, the deleted voice memos will be automatically cleared, i.e. removed forever, from the Recently Deleted folder after 30 days. But you can also customize it by yourself:
Step 1. Navigate to Settings, scroll down to Voice Memos and click on it.
Step 2. Click on \\"Clear Deleted\\".
Step 3. Choose among these 5 options based on your own preference: Immediately, After 1 Day, After 7 Days, After 30 Days, Never.