The whole point of 'earthing up' potatoes is to promote as much growth of potatoes along the stem while also protecting the potatoes from being exposed to light.
If the growing potatoes are exposed to sunlight, they will produce the toxin solanine and become inedible.
You know that spuds have been exposed to light because they also turn green due to the production of chlorophyll.
I start earthing up my potatoes when the leafy bits are just under 12 inches tall, making sure I leave some of the leaves exposed to the sun so they can carry on photosynthesizing - so not completely covered.
I then do it once again a week or two later (depending on how fast they're growing) essentially until I run out of soil.
The potato mounds in my garden are about a foot high or so.
Another way to ensure your young potatoes aren't exposed to light is to use black plastic to cover the soil, although this method requires you actively water your plants as the rain can't get through.
A benefit of using plastic is that you have to do very little digging to get to your potatoes.
This year I used a combination of earthing up and putting a thin layer of newspaper and grass clippings over the soil.
We'll see how that goes...
gardening.stackexchange.com/users/133/Shanna
Growing potatoes in mounds is a great way to grow them because it saves you from having to dig into the soil to pull up your potatoes.
Potatoes never grow any lower than where you planted your original seed potato, but you can grow them as high as the growing season will allow.
There are methods for growing potatoes up columns of tires which you might want to investigate.
Someone else will have to answer definitively how high to mound them, you should be able to cover most of the plant without hurting it.
gardening.stackexchange.com/users/10/Peter Turner
What I do is I take an old garbage can, and I put a layer of soil at the bottom and open a bag of taters and put them in a circle.
Each time I see some leaves
I put more straw on top until I can't see any more leaves.
I water daily.
I do this until the straw/leaves reach the top of the garbage can.
I did this for the past two years and have had no issues.
I'm curious if this is a preferred method?
FYI
I call this my "tater pit".
gardening.stackexchange.com/users/11419/Zach Smith
If it is early in the year and there is a chance of frost, you would cover them as soon as they poke through.
Another good time to cover with dirt is when you can see the potatoes poking through,
this promotes more potato growth, but also prevents sunburn on the new potatoes forming.
gardening.stackexchange.com/users/19366/Annie
Most people plant their potatoes 1-3 times the potato diameter deep and "earth" or "hill" them as they poke through till the flowers come.
This year I had a few left over and planted them in 3's in a 24" conainer.
I covered them with 4" soil and as they poke through
I'll add soil to the rim which will give me 12" soil over the seed.
I am watered regularly and let's see if the yield is any better or worse.
gardening.stackexchange.com/users/21924/robertfauchon
I've heard that I should be piling dirt onto my potato plants as they grow so that I end up with little mounds.  My potato plants have started to poke through the surface.  Should I totally cover them?  By how much?  What is the purpose of this, anyway?
When should I pile more dirt onto a growing potato plant?
gardening.stackexchange.com
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gardening.stackexchange.com/questions/286
gardening.stackexchange.com/users/66/lukecyca
You can earth them up once they are 12 inches tall, leaving a little exposed at the top. It's a good idea if there is still a chance of frost. Most people keep hilling them up until they start flowering. A good time to start earthing up potatoesis when the leafy bits are just under 12 inches tall, making sure you leave some of the leaves exposed to the sun so they can carry on photosynthesizing, so not completely covered. If it is early in the year and there is a chance of frost, you should cover them as soon as they poke through until the flowers come.
A good time to start earthing up potatoesis when the leafy bits are just under 12 inches tall, making sure you leave some of the leaves exposed to the sun so they can carry on photosynthesizing, so not completely covered. If it is early in the year and there is a chance of frost, you should cover them as soon as they poke through until the flowers come.