In every event within the Sprint
Frequently as decided by the team, but not getting in the way of work.
Every event is an opportunity for inspection. In addition, a team can optionally inspect more frequently, without having the inspections getting in the way of the work.
Scrum is a framework. This framework helps companies with developing and marketing their products through the use of their programs. Scrum prides itself with being a simple to understand framework. There are five core values associated with Scrum. These include courage, focus, commitment, respect and openness. However, as in most cases involving companies, the company should have a schedule about when they inspect their software programs among other things.
They should be frequently inspected including Scrum. You do not want to have so many inspections that they get in the way of the work. Instead, Scrum should be inspected in every event within the Sprint, but the whole team should make this decision. It should also not get in the way of the product.