Typical PM queries

Hola PM folks, need your views on these- 1. How do you usually quantify your goal or objective it it’s a feature suggestion (from partners or other stakeholders) 2. When do you use design sprints? Do we need it only when building or redesigning a new product or we can use it even when validating a suggested feature? 3. How often do you revisit your personas or user research? Say for a B2B when your customer base is already known, do we need new personas to add a new feature?