Depends on the console, and the track itself.
If it's something like NES or Sega Genesis, i can separate it into different channels, and work one by one.
If it's a very simple stripped down track, i can pin point things quickly and do it straight away.
If it's more textually rich, or there is a lot of ambiguities in the pitch and the rhythm, it takes me a lot longer, in some cases like more than a week's work. That is, actually working on it, so not things like real life work, where i don't have time to spend on it. What i mean by that is, out of a week, i might only spend 2 days working on it. That kind of thing.
The hardest MIDI i had to sequence for PRC was probably the Batman and Robin source for 394. Only because the panning was super difficult to integrate to MIDI. My PRC400 entry was also a pain, because there was a lot of heavy textures, meaning i had to spend a day or 2, just isolating that particular instrument, bearing in mind this is all by ear. I couldn't separate it into channels like the NES or Sega Genesis can. This was mainly down to the sounds themselves, as the pitches weren't very clear. Still had a lot of fun though, i love the Sonic Unleashed OST, one of my all time favourites.
The hardest for MnP is the Sonic Lost World source for 104, but only because it took the longest, it was still fairly reasonable to work with.
I've said a lot, but hopefully it answers your question...