The length of time an average adult could survive in a typical supermarket with access to a tin opener and a microwave oven would depend on several factors, including the availability of food, the nutritional value of the food, and the individual's specific dietary needs.
Assuming there is a diverse range of canned and packaged food items available in the supermarket, an average adult could likely sustain themselves for several weeks or even longer. Canned foods typically have long shelf lives and can provide essential nutrients. However, it's important to note that a diet solely consisting of canned foods may not provide a balanced nutritional intake in the long run.
While a microwave oven can be used to heat up canned or packaged meals, it may not be suitable for all types of food items available in a supermarket. Some foods may require other cooking methods, and the availability of fresh produce, meat, and dairy products could be limited in a typical supermarket.
It's worth emphasizing that the scenario of relying solely on a supermarket for survival is highly hypothetical, and it's always advisable to have a varied and balanced diet consisting of fresh, whole foods. Additionally, other factors like water, hygiene, and medical needs would also play significant roles in sustaining oneself for an extended period in such circumstances.