How long do germs live on surfaces?

It truly depends. Not all germs are the same.

Oddly enough, microbes can live on plates for a long time.  This is what makes restaurants so dangerous.  Most plates in a typical restaurant have particles of sh*t and semen on them.

They can survive from some seconds to some days depending on the type. 

This depends on the type of germ, the surface material, temperature, humidity, etc

as a general rule, germs can live much longer on non-porous materials (metal, glass, plastic, coins and such) than on porous materials (paper, cloth, money paper etc).. but it depends on other factors as well, such as the availability of nutrients, humidity, compatible air and temperature and so many other factors.. but the most important factor is, the exact type of germs that we are talking about because there are billions of different organisms that can be called germs classified into parasites, fungi, bacteria and viruses.. for example, some bacteria can thrive on the palm of your hands indefinitely unless you do something drastic (drastic to them) such as washing your hands.. some bacteria and fungi has the ability to form capsules (like hibernation chambers or life rafts) to extend their life-span when there are no food nor favourable environment available for them to thrive.. so you need to be specific which organism that we are talking about..

since we are living in a pandemic, the COVID-19 coronavirus (better known as SARS-CoV-2 and not to be confused with the original SARS coronavirus (SARS-CoV) is said to survive longer in humid, in the shade (far from direct sun light) and in moderate temperatures (not too cold and not too warm).. next, coronaviruses can last few days 3 to 4 days on non-porous materials (again like metal, glass, plastic which translate into door-knobs, the handles of stairs and escalators, the buttons of a lift, coins etc etc..), and some say that they can survive upto 9 days maximum if the environmental factors were perfect (away from direct sun light, away from wind, perfect temperatures and humidity etc etc..).. coronaviruses can survive on porous materials (paper money, cloth, paper and such) for few hours, 2 to 3 hours, and upto 2 days in perfect conditions..as a general rule, viruses cannot survive outside living organisms for long, those perfect conditions are not very common, and they often have few hours to live outside a living host, like animals, humans, plants and other germs.. viruses are extremely vulnerable against soup and water (they work better than hand sanitizers), just make sure you sing the Happy Birthday song twice when you wash your hands with water and soup