To what extent is maths talent innate?
Intelignece is a combination of nature and nurture, like most human attributes. The exact percentage is not possible to measure, at least with todays knowledge.

Extract from the article by Neill, J.T. (2004). "Nature vs Nurture in Intelligence" :


  In the overfocus on nature vs. nurture issues, the attempts to
  estimate the relative contribution rests on the somewhat naive notion
  that there is a constant, true value. In reality, "gene expression is
  environment dependent" and it impossible to obtain pure estimates of
  genetic vs. environmental contribution - one could not exist without
  the other.