How much would a person need to weigh so they would start sinking when they stand in grass?

The other examples about weight for construction are about avoiding long term settling of construction, not a single creatures weight. The overall point about weight per square inch are relevant, but let's take a simple example.

Cattle have a lot of weight on four feet that are smaller than a human foot. When they walk, one or two feet will be off the ground, and they don't sink unless it is muddy. Even then, it isna manner of inches and needs to be really wet. A horse running is putting multiple times their weight down and sideways on the ground and their feet aren't sinking in unless it is muddy.

An elephant has a ton of weight but a larger foot. Again, this relatively small footprint per weight does not let them sink into the ground unless muddy.

So if you are wondering how heavy a human sized character would need to be to sink into dry soil in a noticeable time frame while standing still then the weight would need to be many, many tons.

Is many tons more or less than 10?
- Pretty sure a male African bull elephant comes in at 10 tons, they don't sink. But your character, assuming it's bipedal, wouldn't be spreading that weight across quite so much surface area.
  
  tl;dr: 10 tons is still pretty negligible.
- Yes
  
  Depends on how far the character considers sinking.

Depending on the soil, as little as 135. Our back yard is notorious for being soft and springy, and one of my exes weighed that and would sink in slightly.

Btw, the average person weighs more than that because they're average. The question makes more sense if you just ask how much a person has to weigh. It's hard to find a single, worldwide average for all humans, but the range is from a low of about 130 to a high of about 180, which includes only adults.

In our yard, I've seen a 60lb bag of sand sink in and leave a barely visible depression after a few minutes.

What kind of soil you got?!
- Pretty much just some foothills loam. Lots of good organic matter, on top of clay.
  
  On this end of town, we get a lot of runoff, so it stays damp, and I keep things pretty packed with critters by virtue of planting lots of native stuff. So there's a shit ton of worms aerating, we get chipmunks, voles, and the occasional whistle pig. Squirrels love burying shit too.
  
  So the top six to eight inches is almost like well churned potting soil. Just this black, rich soil with occasional pebbles.
Edited the title (question)!

Er - I also must ask that what weight measure are you using?
I was looking for kilograms 😅
- Pounds lol. 135 is a looot of kilos, but fairly normal pounds.
  
  135 works out to 61.12 kilos in my conversion app.
  
  Rough guesstimate, I'd say someone as little as 50 kilos could sink into our soil, it would just spring back almost immediately. I know my niece doesn't leave a visible dent even when stomping, and she's around 40 kilos
  
  But I've been on plenty of grass lawns that didn't give in under my feet, and I'm about 120 kilos (I'm a hoss, in American terms lol).
  
  I've also been around a couple of guys that were over 400 pounds, about 180 kilos and higher. On typical soil, they didn't sink in visibly. Even playing football (handegg version), on fields that are fairly loose soil underneath, I never saw anyone actually sink in, just standing. You'll leave marks when you run, but not just standing.
  
  However! If you're talking about something more like fresh tilled soil, I've seen my niece sink in to that kind of ground, once nearly to the top of her shoes. Even if you laid sod over soil like that, an adult male of average weight would sink in a good bit before they compressed it enough.
  
  Hell, last year, we had a gully washer of a storm that flooded the back yard. The paths where I have grass were actually flooded where you couldn't see the grass at all. The next day, I went out to check for damage, got to the lowest point of the yard and hit a soft spot. Sank in up to my ankles.
  
  All of which is just to point out that soil condition matters more than weight for what you asked about.

If youre a Tolkien elf, no amount of weight results in you leaving footprints ever. Other than that cant help you but i give this small flavor to all Fae

2000 lbs per sq ft is the lowest limit I found for allowable load. My foot is about 0.22 sq ft, meaning that 440 lbs is the safe allowable load for construction.

For reference, a car tire is ~35 psi which is 5000 lb/sq ft. That puts a 1100lb creature on 1 foot leaving a similar dent to a car tire.

I think your "for reference" might be incorrect. The psi of the tyre is the air pressure inside, pushing against the inside walls of the tyre; the relevant numbers here would be weight of the car, divided by the sum of surface area of all tyres on the ground. Tyre pressure shouldn't factor into that at all. edit: I mean it does, because the flatter the tyre the more surface area is in contact with the ground, but if we're talking about that more maths is required.
- https://static.tti.tamu.edu/tti.tamu.edu/documents/0-4361-1.pdf
  
  Page 47, it's typically 10-20% higher than inflation pressure.
  
  I forgot that the fantasy world was full of pedantics

Depends on the type of soil. This site gives some general estimates:

Soil capacity is measured in pounds per square foot (psf). The load-bearing capacity of soils range from 4,000 psf for sedimentary rock to as little as 1,500 psf for clay and silt. Soils that are a predominantly sand or gravel typically fall in the middle, from 2,000-3,000 psf. In very few instances are actual soil tests done. Most inspectors will know the approximate soil-carrying capacity in the areas they cover.

depends how big their feet are! you'll estimate the bearing capacity of the soil (e.g. 100kPa), then divide weight by shoe area to get their pressure on the ground.