can i let a broth simmer for 12hrs.

thank you all for your attention and support -- the stock came out wonderfully. I strained it,
only once but will do a second if i use it as an end in itself.

I simmer mine for as long as I possibly can; however, I'm paranoid about leaving it on the stove all night. I prefer to do it on a day off and let it simmer while I'm doing chores around the house so that I can keep an eye on it. My average simmer time is about 10 hours. Any less time is just not good enough ;) Just keep adding water to the pot as it evaporates. A really good stock relies on that really long simmer

regarding clarity, I agree with ChefOno about not over-straining the stock. Leave it all in there, especially for general use in cooking and making sauces. Sometimes (and only when I'm trying to impress guests), I'll simmer the stock with a raft of ground meat, eggwhites and more aromatics to make a crystal clear broth.

There's nothing - well, perhaps not nothing, but you get the point - like waking up in the morning to the aroma of a stock that's simmered overnight. I do strain my stocks not necessarily for clarity, though from a professional standpoint that's the gold standard, but rather because I don't like the small particulates that travel into whatever I make from the stock. I strain twice: once through a colander to remove the large pieces (don't forget to set the colander in a large bowl ;0) ), and again through a sieve lined with cheesecloth to remove the fine stuff. And be sure to cool it adequately before refrigerating or freezing. I keep plastic bottles filled with water and then frozen to drop into foods just such as this so as to cool them rapidly.

I routinely start my stocks when I get around to it after dinner and finish them when I get around to it the next morning. No issues and several advantages. As Sarah suggests, better to keep the veg in large pieces. And if it's not obvious, keep a lid on the pot and likely your lowest setting on your smallest burner (the temp doesn't need to be at a simmer, just remain > 160F). I'm not a fan of clear stock, or clarifying stock. That gook at the bottom of the pot is flavorful and I don't like throwing away flavor (especially going out of my way to throw away flavor). But to each their own.

Straining through a coffee filter will work too.

I'm not sure there is such a thing as simmering for too long. But per Sarah Reinersten's point one thing you should do is to carefully strain it through cheese cloth and a chinois (if you have one).

The risk of simmering too long: the flavour can get too concentrated (solved by diluting with water if it's too strong for the recipe), or the bones & mirepoix can disintegrate. The latter is only a problem if you're particularly concerned with having a clear stock and can be mitigated by keeping the carrots, celery & onions in very large pieces.

TL;DR version: no you'll be fine.