How long to keep old documents

I am sorting through old papers and shredding them, and I thought someone here might know how long we're supposed to keep certain things. I found internet articles that give advice on some items, but not others.

Like the mortgage papers on a house I had to give back to the bank back in 1997 when I got divorced, and the bankruptcy discharge papers around the same time. I read an article that said to keep bankruptcy papers in case creditors illegally tried to collect afterwards, but it's been 17 years!

Thanks for any advice.