But is this really something to debate around? You finish the mission. You get karma. You spend karma. You have the skill (or spell) when the next mission start. Unless you are doing SRM, I see no purpose spend time and effort trying to bookkeep how many days it take. Shrug.
That is how I'd run it. I can see campaigns that role-play the downtime/training time of things more wanting some guidelines. In which case totally ignore what's in the book it would be a bad suggestion for virtually every campaign in existence, even if you are going for realistic. Realistic there would be different training times for each skill/attribute, it wouldn't have a linear increase in time based on level etc. In most campaigns I'd say
1. Never set up things so you can't break form the freely, I don't care if its a initiation ritual or hitting the gym. You don't want to make a player have to choose between doing his training or going on the run.
2. Make sure it doesn't take longer than the speed at which you earn karma, by that I mean under the current guideline it may take 12 months and 30 karma to increase a attribute, maybe they spend another 60 doing other things. If you are earning over 90 karma in that 12 months, you are constantly piling up karma with no way to spend it. If you give as few karma as the karma section suggests, then maybe slow advancement works.
As an aside to this discussion, the karma award section advice for the GM seems really really bad to me. 1-6 being the new karma award system, and the number to remember is 5, because everything works on multiple of 5? how many runs do you want your players to go on before they can improve their firearms by 1. Sure mages are like hey woo a new spell, everyone else after buying a specialization or two is just waiting for a ton of runs to be able to do anything. Are the new missions following those guidelines?
I mean lets assume I run a game for a year and have 45 sessions, roughly once a week we miss a few weeks for holidays etc. average karma seems to be 3.5ish based on the descriptions. So 157.5 over the year of near constant weekly gaming. So 31.5 attribute points of advancement. Sounds like a lot but bumping your firearms from 3 to 4 then to 5 cost 9 of that bumping some of those dump stats to survivable levels took another 6, getting a core skill to 7, take another 7, getting your 2nd core skill to 6 takes another 6 a specialization or 2 and you are pretty much out. So after a year of near constant gaming you got a little bit better in your core and fleshed your character out a smidgen. And I'm going to guess most people don't get to play that often. Heck many campaigns don't go this long, I'd suggest most don't. It shouldn't be as dramatic as D&D 5 e level 1-20 in 6 months stuff, but a couple dice in your main is barely noticeable, you will probably notice the lack of the dump stat at least.