Micron and Diegojavbau are pretty spot on m8, its a hard beast to judge..For example I've seen some come into the hobby and it can take them a fair time to really start having it click for them, yet I've also seen some come in and withing 2 or 3 works their creating some amazing work..I feel a lot of this has to do with desire and also some past art experience and some good research, I believe airbrushing has little to do with the actual dot and line technique (Some may disagree with me LOL), there's only so many dots you can paint before you've mastered a dot, normally one page and your pretty proficient, lines may take a little longer though to come to grips with..For me its all about an understanding of how a picture needs to be built up, good understanding of color theory, knowing your paint and gun well to avoid or cover any potential mistakes that will happen at any time in someones experience, no matter how long they've painted for..but saying that its good to warm up with dots and lines every paint session, but I prefer my students to really come to grips of them on an actual artwork, plus its more fun

..You'll get to know paint reduction in time, it can be different each day depending on what your painting or the conditions your painting in so trial and error is a bit of a must there, expect frustration to some degree but also understand that if you over frustrate yourself you only constrict whats possible, relax and have fun with it and don't worry about stuff ups, its just a learning curve we all go through, don't allow yourself to give up, conquer it instead and only those who want to give up do..Those who don't generally will succeed and on your last question...Just have fun with it, learn from every resource you can and expect to have to put in the hrs to get joy back out of it..GL
