He might not go anywhere if he gets a ring in the next 3 years, but if he doesn't I think the best time to trade him is probably about two years into his big contract.
That would give the new team 3 years control of his new contract, correct? It should also give us a hefty return.
To me the best time to trade him is when we probably wouldn't want to trade him, because it would be when his value is highest. Maybe after a year, two years or three years of constant improvement (since we can't trade him for a year now). We would find plenty of takers for him, whether there's 4 years, 3 years or two left. I don't think Brad or anyone would have a specific time to trade him, other than looking at what his production is and his fit with the team, how much he is impacting winning, and whether it is likely to get better or worse over time, and what his next contract post this one would look like, and what his expected production would be then.