Conceptually for every call. In practice, a language can do less, as long as it always calls the correct method.
How exactly it finds that method is an implementation detail, but fast implementations will cache the results of method lookups. For example, https://developer.apple.com/library/content/documentation/Co... states:
”To speed the messaging process, the runtime system caches the selectors and addresses of methods as they are used. There’s a separate cache for each class, and it can contain selectors for inherited methods as well as for methods defined in the class. Before searching the dispatch tables, the messaging routine first checks the cache of the receiving object’s class (on the theory that a method that was used once may likely be used again). If the method selector is in the cache, messaging is only slightly slower than a function call. Once a program has been running long enough to “warm up” its caches, almost all the messages it sends find a cached method. Caches grow dynamically to accommodate new messages as the program runs.”
