What is the maximum distance of single-mode fiber?

Depends on the equipment.

Common enterprise grade equipment can transmit up to 10Gb/s of data over a distance of up to 10Km.

Since you need 2 strands of fiber it would be 10Gb of data in full duplex (or 20Gb real throughput)

I would guess some special telecom equipment can go faster but nothing Ive ever looked at goes over 10Gb

The big difference between single-mode and multi-mode fiber is single-mode has a longer range because it is build to a higher specification. A poor comparison would be Multi-mode is like cat 5 and single-mode is like cat 6