Well, when my daughter used the SNS, it was really decided by whether I felt like using it or not. At the beginning I used the SNS with every feed. When I started thinking about weaning the SNS, sometimes I would start feeding without the SNS, and then find that the baby wasn't latching on, so I would tape the tubes on and start of the SNS. Sometimes, I didn't bother trying without the SNS. To be honest, it didn't really seem to matter what time of the day or how many attempts a day I tried to feed without supplementing. By the time I started thinking about weaning from the SNS, it was still clear that most of the milk going into her was still from the SNS and not from the breast.
So I started just once in a while randomly trying cutting off the SNS every now and then. But it didn't work. Then one day, I noticed that the rhythm of the baby's sucking was steady and constant, not lazy like it used to be, and to me it seemed like she was really sucking on the breast correctly. So I had the feeling that when I removed the tubing for the SNS that this time it would work. And it did! It just clicked one magical day.
So for me at least, it wasn't a matter of training the baby to gradually stop the SNS. It just happened.
I do know that the lactation consultant told me that if the baby drinks say, 15cc from the SNS at a feeding, and then the next feeding drinks less from the sns, that perhaps it is a sign that the baby is getting more from the breast and is getting ready to be able to wean from the SNS. It didn't happen that way for me, however, and I know because I logged the amount of breastmilk consumed at each feeding.