DFW ATCT LCL CTLR USES NON STANDARD VISUAL SEPARATION PROCS BTWN SUCCESSIVE DEPS CAUSING SOME CONFUSION TO THE PLTS.

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AFTER TKOF FROM RWY 17R AT DFW WE WERE TOLD BY THE TWR CTLR TO MAINTAIN VISUAL SEPARATION FROM A B727 WHICH HAD DEPARTED AHEAD OF US AND TO CONTACT DEP. THIS WAS ALL SAID IN 1 XMISSION AND I WAS A LITTLE SURPRISED THAT THE CTLR DID NOT FIRST ASK US IF WE HAD THE B727 IN SIGHT AS THEY NORMALLY DO. WE DID HAVE THE B727 IN SIGHT; SO I ACCEPTED THE CLRNC AND CONTACTED DEP. A FEW MINS AFTER I CONTACTED DEP CTL; WE WERE GIVEN A TURN TO HDG 080 DEGS. WE STILL HAD THE B727 IN SIGHT AT OUR 11 O'CLOCK POS AND JUST ASSUMED THAT HE WAS SEBOUND. AFTER ALL; WHY ELSE WOULD DEP CTL GIVE US A TURN E? WE TURNED TO 080 DEGS AS INSTRUCTED BY ATC; AND THEN REALIZED THAT THE ASSIGNED HDG WOULD PUT US TOO CLOSE TO THE B727. WE HAD TO TURN FARTHER N TO HDG 050 DEGS TO AVOID A POSSIBLE CONFLICT. MY QUESTION IS: HOW LONG AFTER TKOF ARE WE REQUIRED TO MAINTAIN VISUAL SEPARATION AND DOESN'T AN ASSIGNED HDG FROM THE NEXT CTLR INDICATE THAT NOW HE IS HANDLING THE SEPARATION? IF THIS IS NOT THE CASE; SHOULD THE CTLR NOT HAVE AT LEAST SAID FOR US TO 'TURN WHEN ABLE' OR SOMETHING TO THAT EFFECT? I STILL FEEL THAT THE DEP CTLR MADE A MISTAKE WHEN HE TURNED US INSIDE THE B727.