WAKE TURB ENCOUNTER WHEN FOLLOWING A B757 ON THE CRI VOR APCH TO JFK; NY.

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WE WERE ON THE VOR RWY 13L APCH TO JFK AT 1500 FT MSL XING THE CRI VOR WHEN WE ENCOUNTERED WAKE TURB CREATED BY A B757 6 MI AHEAD CONDUCTING THE SAME APCH TO THE SAME RWY. THIS TFC HAD BEEN POINTED OUT TO US BY ATC AS BEING 6 MI AHEAD AND WE HAD THIS ACFT IN SIGHT. AS A RULE; I USUALLY TRY TO MAINTAIN SLIGHTLY HIGHER THAN HVY JET TFC I AM FOLLOWING; BUT THIS PARTICULAR APCH CALLS FOR A MANDATORY 1500 FT FOR THE SEGMENT LEADING UP TO THE CRI VOR UNLESS OTHERWISE AUTH BY ATC. THE TURB BEGAN AS A MODERATE CHOP BUT QUICKLY DEVELOPED INTO A HARD ROLL TO THE R OF ABOUT 40 DEGS OF BANK. I CORRECTED BACK TO WINGS LEVEL; BUT ENCOUNTERED A SECOND HARD ROLL TO THE R -- ALSO ABOUT 40 DEGS OF BANK. BY THIS TIME; I HAD THROTTLED UP TO CLB THRUST AND WAS ATTEMPTING TO CLB OUT OF IT. I FOUND SMOOTH AIR AT 1900 FT MSL. I INFORMED ATC WE WERE CLBING OUT OF WAKE TURB AND THAT OUR APCH TO THE ARPT WOULD BE HIGHER THAN NORMAL. THE REMAINDER OF THE APCH AND LNDG WAS TURB FREE. CONDITIONS WERE GOOD FOR PRESERVING A WAKE AT THE TIME OF OUR ENCOUNTER. LIGHT WINDS ALOFT AND VERY STABLE AIR. I DON'T THINK 6 MI IS ENOUGH SPACING BEHIND A B757 FOR A MID SIZE CPR JET. NEXT TIME I DO THIS APCH BEHIND A HVY ACFT I WILL REQUEST A HIGHER ALT TO THE FAF AND/OR MORE SPACING. ALTHOUGH WE ROLLED ONLY ABOUT 40 DEGS OF BANK; THE ROLL RATE WAS VERY IMPRESSIVE. I WOULDN'T LIKE TO SAMPLE IT AGAIN.