As we have practical and powerful quantum computers, this is very relevant question to be answers in more practical ways not in theory, so how much time the current quantum computer would take to break SHA256 hashing?
There aren't any practical, general purpose, quantum computers in operation, for the moment; but if the claim is that there are, why not just try to ``break SHA hashing'' using one of them and see how long it takes? Though I would imagine that it would be easier to use a network of ``ordinary'' computers...
The 'very' short answer is: There are two types of Quantum ALGORITHMS that can be used to crack hashing functions, namely the Grover's algorithm and the Shor's algorithm. Once you fully understand how they are actually run, you might think about replacing the 'how much time' in your question with 'how many Qubits'. After that, you can search for 'current' advancements and updates on known Quantum computers such as IBM's, regarding the number of qubits available and you can compare required vs. available resources .
best wishes.
What do you mean by "cracking" SHA256? It is not really a cipher nor it uses a key...
Mahendra Kumar Shrivas , Mario Stipčević Nusrath Konnola Muhammad Minoar Hossain Stam Nicolis Motivated by your question (and answers), I posted a strongly related, currently under review paper on Techrxiv. Please take a look at it :
https://www.techrxiv.org/articles/preprint/Bitcoin_Revisited_Formalization_Benchmarking_and_Open_Security_Issues/20496492/1