The priority queue adaptor uses the standard library heap algorithms to build and access the queue – it’s the complexity of those algorithms you should be looking up in the documentation.
The top() operation is obviously O(1) but presumably you want to pop() the heap after calling it which (according to Josuttis) is O(2*log(N)) and push() is O(log(N)) – same source.
And from the C++ Standard, 25.6.3.1, push_heap :
Complexity: At most log(last – first) comparisons.
and pop_heap:
Complexity: At most 2 * log(last – first) comparisons.