The first thing to do when you decide to build a model is figure out how you want to build it. Do you want to build it strictly out of the box, modify it, completely change it, or something in between. You said you've been building models since you were a kid but have never been satisfied with how they turned out. I would suggest start again by building one straight out of the box. This way you can work on perfecting the basics such as:
sanding down sprue edges, mold lines, and flash.
glueing and what types of glue work for the different types of parts.
filling and painting.
You didn't say but are there any specific concerns that you have with your models. It is easier to give answers to specific questions than to just give general guidelines. If you're having a specific problem, please feel free to post a detailed description and we will be glad to give you more detailed help. Also feel free to email me as well if you run into a trouble spot.
As far as how long it takes to build a model...The answer is as long as it takes. A lot of times it depends on your mood. I've had models that I've finished in as little as a couple of weeks and right now I am still working on one that I started years ago. :-) As long as you're having fun, that's all that matters!!!!
Ray