There is a kind of cultural rule for that. It is called the 5 second rule. If you dropped your previous snack and you picked it up within 5 seconds, it is safe to eat. I don't know where that rule originated from but it sounds ridiculous.
Before even talking about the longest time to pick up a food, we should first consider the type of food and the surface the food fell on. There is research at Rudgters University and other centers debunking that rule.
Of course the more food stay on the floor, the more infected it becomes. What sort of food and the surface will determine pick up the food with your hand. A food like water melon that is wet and sweet will likely attract and get infected by bacteria immediately touching the ground or a shining surface where bacteria are likely to be faster.
It is better to let go and stay safe.
As a chef if something I have dropped on the floor while at work it would go straight in the bin.
After a few beers and the lights of it normally would get picked up and eaten when i find it :D hehe.
The 5 second rule like golddeejay mentioned is something I think we have all done in our time and think applies when we are soba.
I have a friend who has eaten food from the floor after it has been there all night, i have seen him do it, we call him the iron gut as he never get poorly and eats allsorts maybe it is because of these behaviours is why he can eat things and not get ill but i know I won't be doing that any time soon.
This brings back old memories. I remember when I was young, I mean much younger, r from about 6 to 8 or 9 years, thereabout. I used to pick up food from the floor when they fell off my plate or from the pack of biscuit I was eating. I never really did this when people were around. I hate that I'd have to lose any piece of food.
There was a saying then that any food that fell to the floor belongs to Satan and that he'd eaten it already. This was would only deter me when people were around. If they were not around, I won't hesitate to pick it up immediately it falls. And there were some mischievous people, kid like me then who would take it upon themselves to crush people's food that fell off. They took it on their heads like government work.
Things changed however when I grew up. I mean, a few pieces won't keep you from being satisfied. I don't do it anymore. Even, looking back at those acts I realize the health hazards I was exposing myself to. Some of those floors had been walked upon by countless numbers of people and there was no way of determining how clean their shoes were or where they had gone to with them. It's childish and shouldn't be left to continue.
I don't agree with the premise of the question. It doesn't make any difference as to how many seconds or minutes the piece of food spends on the ground. When it falls on the ground, it becomes immediately contaminated with micro organisms on the ground. It takes some time for the micro organisms to grow in the piece of food. If not enough time has passed for the micro organisms to grow, then it will be safe to eat because the bugs will all be killed by stomach acids. If the piece of food has spent enough time in conditions favourable for the bugs to grow, then it can be hazardous to eat it depending on the bugs and to what extent they have grown in the food. But what is really relevant is how long the piece of food has been uneaten or unpreserved.
If it is something that dirt does not stick to I would go with the 5 second rule. if it is a sticky food that the dirt would stick on and not come off I would probably just dump it in the trash.