Anyone who has arranged a song for fingerstyle guitar will tell you it is a labour of love. Generally, particularly for rock and pop, you work out the chords. If there is a signature bassline for the song then you'll make a resolve for that...if the bass isn't prominent you might just fall back onto some bassline that fits the genre - or one that you know you can play easily and reliably without thinking! Add on the melody, which generally involves forcing some crazy thumb independence into your brain on the right hand, and then some bizarre fingering solution for your left that you've never used before. Fill it all out with something in the mid, arpeggio or a finger flick, just something to give it a bit of fullness. Sometimes a distinct plucked melody on bassline works fine; the options are endless, the end result is "you".
Initially you are pleased to get out anything, and to be actually able to play it too. Then you start getting increasingly trickier. Yeah, okay, you pat yourself on the back for coming up with an amazing resolve that "no one else could ever have thought of" - don't get too much of a head swell, there will always be the next guy who will come up with a better arrangement than you.
Now this might be coming down to a quantity-versus-quality discussion, or at least a trade-off between the two...but I'm in the final throws of another arrangement and I'm not sure how far to go. For instance, for a standard verse-chorus rock song, each verse is identical except for the singing. You could play the same verse identical and maybe nobody will notice (except for you) that you missed out some subtlety. Some subtlety might be easy and worthwhile, like there are some extra syllables of the some note - not hard to do. The other day I found a verse that dropped one note down a tone in the lyrics...the note forced me to go behind a barre chord, which meant I had to reform the method without the barre, which then disrupted my left-hand right-hand craziness that I had burnt into memory...
A lot of work for one note!
Now I Naudo would do it...however this time I don't think I will. When I look back at earlier arrangements they were verse-for-verse chorus-for-chorus identical. As I started to get a bit better at creating and playing crazy fingerstyle arrangements I discovered I was more able to mix it up a little between verses without any major re-learning/disruption. It's a carefully paced progress...if I try too far beyond my skill level at any time I might lose interest, be left with incomplete arrangements (ha, got a few of them)...if I don't push the boundaries a bit I won't be improving my skill level.
Perhaps there needs to be a time rule. Each arrangement may only take two months from start to fluently playable. The two months may be stretched over a year...noting that "two months" for me means about 3-4 hours a week for 8 weeks on the job at hand.
That way, whatever you can fit in two months is what you should aim for. To start with, it might not be a lot. At the end, in two months, you have captured every subtlety, solo, inflection, nuance of the song!