Being somewhat conservative, recently in the US there have been an average of around 75000 new cases of covid reported per day. Let's assume there's a 14 day window of infectivity (in reality this period is shorter but there are also unreported cases) so there's a little over 1 million people in the US who can infect surfaces at any given time. Pretending for a moment that these are evenly distributed, that's a 0.3% chance of any given person having covid. If a surface remains infected for 3 days, that would mean that 230 people would need to come into contact with it every 3 days for it to have a 50% chance of an infected person coming in contact with it. This would average out to 77 people per day. If 1000 people traffic a location over an 8 hour day, the odds that an infected person has contacted the surface in the past hour are around 31%. Now the odds of a an infected person actually contaminating a surface they touch are probably lower still, but this would seem to indicate most surfaces in public places have significant likelihood of being unsafe.