I have been hearing and reading so many cases on giving up playing piano and ended up having to practise more in order to go back to the past standard...

So this really conclude that once you stopped practising, you will lose some music skills???

Or does there anyone who can pick back up music with no problem?

Yes if you stop playing you do lose some of your musical skills. For example my flute playing is nowhere near as good as it was 10 years ago because I don't put in the practice. I can pick up the flute and still play the pieces that I used to play but my tone and technique are not as good as they were. I think it would probably take me about 6 months to bring me back up to my previous standard, with lots of work on tone/technique exercises. However, at the moment I am really focused on learning the piano and know that I have not got the time to put in lots of work on the flute so I just play it from time to time for pleasure.