Denim is graded in by weight in ounces. This means its weight per yard of fabric at a 29 inch width. But keep in mind most shrink to fit jeans will increase in weight when washed, as the fabric contracts.
Early Levi's jeans were around 9oz, which has since increased through the years. Most modern jeans are around 11-13oz, but you can get them a whole lot heavier.
There is one particular company, that has taken pride in producing some of the heaviest and toughest garments on earth, and that company is Ironheart. Their regular jeans are 21oz denim. But they aren't producing the heaviest jeans on earth. At the moment that record is being held by the Canadian denim company Naked&Famous with a pair of 32oz jeans, that could stand on their own (you don't need to 32oz denim to see jeans standing on their own, by the way). Now, I'm glad I don't have to wear that pair of jeans. I don't think it would be very comfortable, but then again once they are broken in, I think you would be covered for a good amount of time. Naked&Famous is a very untraditional denim company. They produce everything to a high standard in Canada using some very special, cutting-edge fabrics. The 32 oz denim, which is probably only usable in bags is a good example, so are the jeans made of glow-in-the-dark denim and cashmere denim.
They do not look particularly well-sewed, but I read that they were quite a mouthful for the company, that was contracted to sew them. They put their machinery to the test and I think a lot of sewing needles were lost in the battle.