Probably not as deep as you’d think.
Many companies will spend the majority of an interview drilling in deep on a candidate’s technical ability, and spend comparatively little time assessing their interpersonal skills, which are arguably more important than the technical skills (especially at the senior IC level).
You can pretty easily get a sense of a candidate’s ability in about half an hour. The rest of the time is typically spent finding the depth of their knowledge in various areas. So how deep should you go? As deep as you need until you’re comfortable making a yes/no decision. Some people can come to that in an hour, some need a few hours. It all depends on your team culture and the specific needs of the role.