For example, if Better Learning V will take 9.5 days to complete as it is, what's the most speedup can I get with favors from the giants? Is there a formula? Ten favors = one min of speed?
Most you can speed it up by is half the original time; favor makes the Rock learn at double speed for X seconds (don't know what the exact relationship is, but more favor means double speed for longer.)
It depends: Primary giants give the most, secondary ones give the second most, and all others give the least amount. I haven't bothered trying to calculate the exact numbers. I also seem to vaguely remember reading somewhere that the maximum you can decrease a skill learning time is by half of the original time...
Also, some advice to maximize the amount of time you can knock off of learning a skill:
1) Spend favor points (doesn't matter how much) to decrease the skill time immediately after starting to learn the skill. This gets it going into accelerated learn mode right away, which will help to maximize the total amount of time you can knock off.
2) Maximize it even further by making sure you keep the skill in accelerated learn mode all the time. Don't let it slip out of this mode until you are told by the game that it's learning as fast as it can, and that you can't spend any more favor points to decrease the time.
3) Each game day (every 4 real hours) you can gain a maximum amount of favor from each giant equal to the amount it would take to get a new emblem when starting at zero favor. Each game day, start with the primary giant for the skill, donate up to 1 favor point before you hit the max (or you'll get an emblem and won't be able to spend the favor on skill learning), and spend all the favor on your skill. Then, do the same with the 2 secondary giants. If you like, you can do this for others as well, but it will cost more favor points for the same amount of skill learning time reductions. At new game day, repeat this again. Keep doing this until you're "learning as fast as you can", and the game won't allow you to spend any more favor points.
IIRC, you cannot suddenly finish learning a skill simply by applying enough favor points. You can only lower the amount of remaining time, and the limit on that is one-half the remaining time.
WindBorn is correct, and Shepherdmoon's advice is spot on. You can reduce up to half of the remaining time on a skill: if you start learning the skill faster/spending favor the exact second you start learning it and never let the fast learning run out, this can be half of the original total time on a skill. If you have a 2 hour time on a skill and wait 1 hour to start spending favor, the most you can remove is 30 minutes.