thunder wrote:How long does it takes to develop MPC 500? Time, from start to a point, when you can say "I know all options, can use them easily and quickly.. And when I have any idea of new song, I just make it on my MPC"?
Year?
I mean,.. if you listen rap music, but decided to make own.. and bought MPC. Real beginner 
If you listen to rap music, then decide to make your own, and never made music, let alone sampled a thing before, then you have many mountains to overcome.
Learning the machine is only a small part of the process, but if I had to say, being a complete noob, and starting from scratch, with no prior beat making knowledge...
It is going to take a while before you will be able to use the MPC at its full potential, like a paintbrush.
If you have no sequencing knowledge, and no way to visualize what is happening when you hear a beat, it is going to be a tough process at first.
Once you begin to understand what is actually happening behind the scenes when you hear a beat, and can picture it in your minds eye, it will be easier for you to take an idea and just SLAM it down.
Either way, if you are REALLY passionate about it, you will find yourself choosing beat over food, sleep etc.. until you learn it.
In that case, year! Otherwise, couple, maybe few, possibly never...
I know how to use at least 90% of the features, and was making beats an hour after I learned to clear the memory. Had it running in sync with Reason, and the Motif the next day... But I also have an extensive history of sampling, sequencing, and making beats...