I'll put forth that nothing of this sort, from the semiconductor age onwards, can be an artifact as we have far more readily available and detailed records to work from.
Such graffiti is only an artifact when it's purpose, history and other aspects are lost to time due to lack of alternative documentation.
In effectively all cases these days there is simply no additional value obtained from graffiti like this, unlike that of street artists doing murals or painting trains.
I believe it is 50 years in the US. I can't find it on mobile, but I remember reading an article a while back about some simple "john was here" on some anasazi petroglyph that hadn't been removed because it was from the late 1800s or early 1900s and was considered historic.
Such graffiti is only an artifact when it's purpose, history and other aspects are lost to time due to lack of alternative documentation.
In effectively all cases these days there is simply no additional value obtained from graffiti like this, unlike that of street artists doing murals or painting trains.