I don't have a glass railing, but to figure out how often you need to clean it, think about how often you clean your windows. The exterior glass railing will get dirty on both sides, while your windows are only exposed to the outdoors on one side.

I have it around my pool I clean them once in the spring and one more time during the summer I do not however use any window cleaner that BTW actually gets dirty faster I use micro fiber cloths and vinegar water . I do find I use unsoftened water it seems to take longer to get dirty . So we have our cold water not softened so I just heat it on the stove . I have a house with many large windows and they get dirty inside and out just need cleaning too. I guess it depends on how much you love the look which is my first concern since I am aclean freak anyway. I love the sparkle of clean windows