It could take 20-30 hours. Depends on your team, and how fast you get into the portals.
Spend the zeal when training the skill start to take too long. Of course, the higher level you go, the better and the more you would get out of the zeal. But it's not that big of a difference if you level it up a few times.
2/3 days without no-lifing, you should spend it once you get to 90+ or high 80's in that skill otherwise it'll just be a waste and always spend 100 zeals at a time so you get the 10% bonus too