Is the Big O notation used for time estimation only ?
Its not that it’s used only for time estimation, but we are taught for better understanding in that way. We have seen at many places like the space complexity is O(n) which just means that maximum space we need to allocate is n blocks. So, it defines upper bound of an algorithm, can be both time and space. Well clear from this line:
Wiki says, Big O notation is used
to classify algorithms by how they
respond (e.g., in their processing
time or working space requirements)
to changes in input size.
Your answer lies in bold characters of the above wiki content.