Student or Learner
As the title, what may be the proper question to the reply sentence?
"How long have you learned to play the piano?"
"When have you learned to play the piano?"
Thanks a lot.
"When did you learn to play the piano?" could be answered with "I've learnt to play the piano [only] since I entered college". However, the more likely answer would be "I learnt to play the piano after I started college".
Bhaisahab's question is much more likely.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.