Hey, I'm a bit late, but it's 57-bits. On 64-bit systems page tables have 512 = 2^9 entries and page frames are 4096 = 2^12 bytes long.

So the number of bits in an n-level system is 9n + 12.

9x5 + 12 = 57

On 32-bit systems the number of PT entries was 2^10, which is why they could have a 2-level system. 10x2 + 12 = 32.