If you're truly skinny then seeing some packs won't take very long. Are you skinny to the point where it's REALLY hard to pinch anything on your stomach? Sometimes people are skinny but still have more body fat then it seems they do. Just do a ton of different ab exercises and only rest a little bit between sets, maybe 20-30 seconds. I used to work abs for 30-40 minutes sometimes everyday or every other day and do a lot of cardio and I started seeing a lot of definition in about a month-1 month and a half. Given that I was decently skinny to begin with.