How long would it take to know if I have done damage to my car from driving without oil?

Of course it's damaged - impossible to know how much without an expensive inspection. . . . . so it might blow next week, or next year. . . . but it won't last as long as it would have. One of the disadvantages of buying a used car: not knowing if it had average maintenance, extraordinary maintenance - or was abused like yours.

I had one of my cars spring an oil leak on the freeway once when I was a kid and it ran dry quickly, I had to replace my engine because it had irreparable damage. How dry was your engine exactly, was it only 2 quarts low before it showed up as 'full' on the dip stick? Most cars hold 5-6 quarts of oil so if you were only 2 quarts low and still had 3 or so in there you might be OK, kind of hard to explain how bad it was running unless it was lower than that in my opinion. If the engine was completely drained, not just a couple quarts low I am surprised it's "running fine" now, or at all depending on how long the car ran without any oil. It's hard to imagine it was completely dry though because it likely would have seized up very quickly and then failed to start again. Good luck, definitely locate that leak because you never know you might be lucky... depending on just how low it was. You said you only added 2 quarts after all...

if you have good oil pressure and it is running quiet, just keep it full and don't worry

If its a knock sound it's the rod

Keep the oil filled. Drive another few days if you can keep the oil level normal. If it seems ok... Get a price to fix the leak. Awful noise could have been the hydraulic lifters starved for oil.

Yes, its damaged, especially if you say it made noise, .....How seriously,..... only an engine inspection can tell for sure