I've only done shorter stretches of the JMT, so I can't say. Your 16 days is neither the fastest in the world, nor is it just loafing along. Until you get a better water estimate in mind, why don't you use one gallon per day (above what you boil for cooking)? I've used that figure in other places. If you go in early season, it will be cooler weather and slightly less need for water. If you go in late season, it will be cooler weather and fewer streams to get good water from. If I had to make a wild guess about my own water consumption, it would be 2 to 3 quarts per day, outside of boiled water. But that is just me.
Since you can't guess about your water consumption, I will offer one suggestion. Dose your water with some Gatorade powder or similar electrolyte/energy drink, but don't make it full strength. Some people drink only straight water in large quantities, and then they are heading off the trail to visit a bush every 45 minutes. That's OK, but it is a little unnecessary. If you get some electrolytes into the water, your body's muscle tissues will tend to retain the water longer, and you end up drinking less water and visiting fewer bushes. If you overdo that, your urine will become bright yellow and will be infrequent, which is a sign of dehydration.
If you are experienced enough to be doing the JMT, you should probably have a handle on your expected water consumption before you leave home.