Skill XP alien XP human gain %
Piloting 15 13 2 13.3
Engines 15 13 2 13.3
Shields 55 50 5 9.0
Weapons 65 58 7 10.7
Repair 18 16 2 11.1
Combat 8 7 1 12.5
You can easily check the amount of XP required to level up a crew member by hovering his skill icon; the data from wiki is 100% consistent with both in-game values displayed and my playthrough experience. The phrase 'humans require -10% XP' is a quite good approximation, since in most cases the XP got reduced by only a little bit more than 10%.
The rounding and exact values are irrelevant here - just assume that humans require less XP, roughly about 10% less; since XP is quantized in FTL it'd be impossible to get exactly 10% XP reduction on any of the skills.
BTW, they aren't learning faster or anything, as the learning rate is fixed and luck-independent with 100% rate as far as the triggering event happens. Some events are harder to come by (e.g. evasion early, shield hits and repairs later on if not skill farming explicitly), but the XP gain happens in exactly the same way anyway, for humans and aliens alike.
Side note: while humans require slightly less XP, this bonus is usually worthless since you can easily farms skills, especially in later game, not to mention how small this "bonus" is and that humans have the least amount of blue options for any races (only the 'mantis thinking he's a human' one AFAIR). Overall, this IMO makes humans completely useless in any playthrough above Easy, especially when compared with Engi's blue options, Zoltan's free energy and Rock's HP, fire resistance and blues - having exactly one Slug is a nice thing from time to time too; boarding ships benefit from 2 Mantises greatly. My usual Z8 crew consists of 4 Zoltan, 1 Engi, 1 Slug, 1 Rock and 1 Mantis; Since getting 4 Zolts is a bit tricky & luck-dependent, I replace missing ones with Rocks, Mantises, Slugs, Engi (never more than 2 at the ship at the same time!) and Humans, in this exact order.
Humans have only a negligible boost to levelling skills, making them almost completetly useless overall.
http://ftl.wikia.com/wiki/Races/Human , playthroughs.