5km is the IEEE standard for 1000BASE-LX over SMF.
10km is what Cisco and some other vendors will support with their GBICs.
You can go beyond 10km with Cisco's LX/LH GBICs. How far beyond that YOU want to support, depends on the quality of the fiber, number of splices, and connectors.
I have personally tested and used links that worked at 16km. And I have seen links that did not work at 20-21km. That's with Cisco LX/LH GBICs at each end. (The 20-21km link worked with ZX GBICs and attenuators, however.)
The minimum optical power budget for a WS-G5486= LX/LH GBIC is 9.50dB. If you can measure your SMF cable plant with an optical power meter and the total end-to-end loss (including connectors and splices) is less than or equal to the min. power budget, then it will work.
Remember, for the optical loss measurements to be valid in determining whether you can use LX/LH GBICs, you must use a 1300-nanometer light source to do the testing. The attenuation characteristics of single-mode fiber at 1300nm (LX/LH) are different than what they are at 1550nm (ZX GBICs). Specifically, the dB loss per km of fiber is anywhere from 30-75% higher at 1300nm compared to 1550nm. So make sure you are not using test results that were taken at 1550nm to decide whether or not it's feasible to use 1300nm LX/LH GBICs. (I made this mistake once.)
Having said all that, the longer the single-mode fiber or the closer to the max. distance you are, the less margin you leave yourself for future errors in the cable plant if you choose to use LX/LH GBICs. Some of the things that can chew up extra dBs of optical power over time are: additional splices to repair breaks; sharp bends in the cable; natural degradation of the fiber over time; dirty connectors; etc. To be safe, either leave yourself a 1-2dB cushion when using LX/LH GBICs, or upgrade to ZX GBICs.
