It depends entirely on the synthetic oil in question, they all are designed differently. Even the cheapest synthetics are usually good for about 7500 miles or 6 months , then you can get all the way up to the premium Amsoil that is good for 25000 or 1 year ( this is without bypass filtration, with bypass filters you can go a LOT longer and only considering diesel oils and our engines you could go 50,000 with the same oil in some engines but they had longer drain intervals to begin with, and oil filters must be changed in between time, normal oil filters are only good for about 6000 -7500 miles max before going into bypass, Donaldson filters will hold out for 12,ooo, Amsoil filters will hold out for 25,000)
Now, is that clear as mud?