[hilbert42]

Addendum: I should have added that from various sources, Wiki etc., it seems the W- boson does decay very significantly faster than the complete decay but we've still no further info about the timeline of events, as you say, it's likely not possible.

In hindsight, it seems the techniques mentioned in those links to measure a particle's time are unlikely to be applicable or adaptable here. That then begs the question about how did we initially determine that the W- boson's decay is much faster than the overall process.