I'm not sure what the exact time frame is from when a subway poster goes up to when it's defaced, but I'm going to guess it's somewhere around 90 seconds. The general graffiti go-to is mustaches and ding dongs, but occasionaly the metro vandals like to get a little more creative. For example...

 

1.

subway poster graffiti

Source: basenow

2.

subway poster graffiti

Source:  businessinsider

3.

subway poster graffiti

Source: @thesquirrelsnest

4. 

subway poster graffiti

Source: pintrest

5. 

subway poster graffiti

Source: imgur