Well, it took me 6 years to learn that, haha. No, just kidding, but many Russian kids do actually have problem with that.
It will take you from one hour to infinity - it depends on how hard you want to learn it. I don't think it's impossible for English native speakers, and you can find many tutorials on youtube. You can try those for Spanish and Finnish too, as the sound is identical. You need to find the way to pracrise it convenient for you. Try this, for example: http://www.youtube.com/watch?v=XKRQMCHlONU or any other videos named "how to trill", "how to roll R". It's fun!
It's individual. O.o
I'm native speaker and I can't prononce P ;) and I never heard that somebody not understand me. lol.
Childs spend 2-3 month to learn P and use it in words. I think the same is true for others.