Did a quick test with Reaper stepping through the first 8 factory presets with my FC-12, and I'm measuring about a 75 ms average drop out for preset changes. Each of those has between 12 and 14 blocks in them, so that's probably a fairly decent average for most preset changes.
In comparison, channel changes for the amp block take about 30 ms but are more fade out and in than a full silent drop out.
As expected, presets with more blocks take longer to load. A preset with only an IN1 block, a full row of shunts, and an OUT1 block loads in about 20 ms, while one with just an IN1 block, Amp block, Cab block, and OUT1 block loads in about 45 ms.
In these tests, spillover was turned OFF in the Global Settings menu.