I think it will take you even more time to upgrade to 20, every new upgrade becomes more and more expensive, after level 15 it goes insanely expensive. It takes 8h to upgrade explorer skill, lvl 20 upgrade had a cost of 191 uranium so you can imagine. If you have 120 uranium production per day, it would take 2-3 days for one upgrade, when you push for explorer skill you also cant upgrade anything else. It's a grind but once you get to 20 it starts to get very interesting.