An old rule of thumb was about 10% of the time, but ideas on that are all over the map.
Actual instrument time is that time spent operating by reference to instruments. Whether or not you're under IFR is irrelevant as you can be in visual conditions or instrument conditions while operating under VFR or IFR. Dark night over the atlantic, good chance you're in instrument conditions, and operating by reference to instruments.
That said, I never logged any of that time as actual instrument, given that it was invariably autopilot on and I wasn't really doing much beyond making the occasional radio call and monitoring. On a 7 hour leg, I might show a few tenths of actual instrument, even though the flight was conducted in what were unarguably instrument conditions.