Looking for a new laptop, I would like your recommendation: Dell Insipiron 15 7000, Asus N550JK (Or any other Asus), etc.

First off, don't touch the MacBook Air, sure out of all of these it has the BEST battery life (roughly about 11 hours if you get the 2014 version). The Lenovo Y50 would probably have the best specs overall if you want a powerful gaming laptop for under $1000. The ASUS and Dell just depends on the specs you are going with them for.

If you want a longer battery life though, the Dell Inspirion with the i7-4510U with the GTX 750M (as listed on dell's website isn't that bad) is ok, or on lenovo's site, switch down to the Lenovo Y40 which is an inch smaller, but will use the U-series processors for better battery life.

Unfortunately the Asus variants for that model don't really have a model that has the U-series processor. And the Lenovo U530 doesn't have a graphics card listed by Lenovo except for the Intel HD 4400.

I'd say for the Y50, ballpark of 5.5 hours, the smaller Y40 will get about 7.5 hours and supposedly the Dell can get 9 hours just doing web surfing and etc. But battery life is inversely proportional to its graphical performance too so take that into consideration.