It depends on your pace mate. But the vast majority of runners should get nowhere near 20 miles when training for a marathon.
The evidence tells us that, once you go over 12 miles (yes, just 12), the benefit declines rapidly. There is some benefit but it's relatively negligible. And once you run for over 3 hours, you actually enter a zone of detriment - your fitness will worsen. This is because there is essentially no fitness benefit beyond this point but your body will take a significant amount of time to recover. This, in turn, will hinder your other training runs for the rest of the week (and potentially beyond). The long run is the most important run of the week, but it isn't so important that it should be to the detriment of your other runs. Aiming for a high cumulative weekly mileage with one of your runs as an LSR is the key to successful marathon training. This can be further supplemented by optional speed work and strength training.
So to look again at maximum LSR: aim for no more than 3 hours on feet in one go. This doesn't mean that you should be at, or close to, 3 hours every week though. Let's look at an average runner whose LSR pace is 10m/m. In three hours, they will run 18 miles. To have a faster LSR pace than this, you're typically looking at going under 3:30 for the marathon. So it will take a very fast runner with an LSR pace of 9m/m to cover 20 miles in 3 hours.