A Self-Explanatory "S"

On the little dead-end side street by my apartment there are a surprising number of markings, even though I doubt any new development is even being considered back there.  On a walk this afternoon, I saw a series of these "S" symbols in green, 

 

so of course my curiosity was piqued.

Though less than 20 paces later the explanation came up in a very obvious way.

So, yeah, S marks sewer lines around here, and across the street down at the bend were some "No City Sewer" markings.  I wondered about the choice of that sickly shade of green.  Then 20 paces the other direction there was this huge but faded "X marks the spot" around what looks like an old surveyor's mark that had been pounded into the pavement.

This made me wonder: do these marks ever get cleaned up or removed after the work is done, or do they just fade away while other markings are added years or decades later?  How long would one of these spray-painted things last?  Other faded tags appear to be all over, and then fainter ones, and fainter ones, then ones that are so faint they're probably even there.