Which and how much mathematical operations are required for the blockheader-to-blockhash operation and how fast would a human be able to calculate a single hash?
EDIT: For simplicity, I think it's a good idea to use the world record for mathematical calculations to make it a little more fair.
The user C121 on r/bitcoin explored this topic in the thread Mining Bitcoin by hand.
He states that it takes 3385 integer operations to calculate one double SHA-256 hash.
His conclusion was that you would reach about 0.00003 H/s, or in other terms, it would take about 9.4 hours for one hash, assuming the human in question could do a 32-bit operation in 10 seconds.
He also estimates that you would fill about seventy pages of paper with the calculations per hash.
Update:
As Mathias711 pointed out in the comments, Ken Shirriff actually calculated some SHA-256 operations manually and wrote an article about mining bitcoin with pencil and paper. He arrives at an estimate of 0.67 hashes per day, at a vastly uncompetitive energy consumption. ;)