Takes minutes to learn, forever to master, you can write a song with a synth in a couple days with no training if you're fine with trial and error. The most important step is to take the first step, start writing shit, just keep writing shit until you can polish it into something awesome.

You mean piano?

It depends. A smart person who works hard can be good in a year.

Depends if you mean playing keys or just creating sounds. Many people who play synth are usually not well trained piano players so learning piano is not a big deal with synths, but you will sound so much better if you can play piano well.

As far as creating sounds, it's not that difficult. You just start turning knobs, but it's better to understand what the knobs do. Learning how subtractive synthesis works is a first good step. This is the most common type of synthesis found in synthesizers.