On our small urban farm we've been busy collecting black walnut sap. After about 2 weeks of collecting sap (from 10 spiles), and 20 hours of boiling it down, from a collection of about 120 litres of sap down to 4 litres of delicious, ultra sweet black walnut syrup - and our first pancake breakfast with fresh homemade syrup - we think that's not too bad, and definitely worth the effort!
When a friend asked how to know when the sap is syrup (so as not to keep boiling too long and get taffy or maple sugar!) we found that it becomes fairly obvious - the bubbles become brown and foaming, and it's important to watch that the pot doesn't boil over. We tried to capture a photo of this in the 3rd photo down below. For the first 15-18 hours of boiling there was still a lot of water evaporating so if possible this is a good stage to do outdoors (or with good ventilation or kitchen range hood fan going). For the last hour or so it goes much more quickly. With lots of stirring needed at this point so the near-syrup doesn't burn on the bottom of the pot, and a candy thermometre handy to read the temperature, the sap has become syrup when it reaches 219F. At that point, pour into hot sterilized (clean) mason jars, scrape out the pot and lick the spoons! Then start a batch of pancakes. Yumm! We keep a good stockpile of syrup in our fridge and store the rest for longer storage in mason jars in the freezer. This keeps well for many months.