When it starts to show signs it needs it. As long as you properly rinse it, there is really not much to service. The only real things to "service" are the inflator and any dump/overpressure valves. The inflator starts showing signs of sticking or leaking long before it's a big problem and even then, you just detach it and keep diving. Leaking OP and dump valve show up as slow leaks - BC will slowly leak down and most of the time, it's just sand or other trash in them. A quick disassembly, cleaning and reassembly is all that’s needed. That said, a good once over before a dive trip is always a good idea but that should be part of you pre-dive planning anyway. Check the inflator, deflator, OP and dump valves and does it hold air for an hour or so, if it does, it's good to go.