What is the cost of len() function for Python built-ins? (list/tuple/string/dictionary)
It's O(1) (constant time, not depending of actual length of the element - very fast) on every type you've mentioned, plus set and others such as array.array.
Calling len() on those data types is O(1) in CPython, the most common implementation of the Python language. Here's a link to a table that provides the algorithmic complexity of many different functions in CPython:
TimeComplexity Python Wiki Page