how many years for heat generated in the sun's core to reach its outer layer,the photosphere?

Answer 1

Nobody really knows, but it may be as much as 50 million years or as little as 17,000 years.

Answer 2

Campbelp2002 pretty well sums it up.

I found an article (see link below) which begins with:
"Asking how long photons take to migrate to the sun's surface can start a fight. Some solar physicists say 17,000 years, others 40,000, some 170,000, still others 1,000,000 years. It's a fairly tricky question, since the sun's core — where a photon starts — is a bad place to make much progress."

We had the same question in my astronomy class and we had, let us say, a lively discussion (fortunately, no fighting).