ATR42 FLT CREW IS GIVEN TKOF CLRNC BEHIND A HVY JET PRIOR TO THE REQUIRED WAIT PERIOD FOR WAKE TURB.

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I WAS FO. I WAS ACTING AS PNF; THE CAPT WAS THE PF. RWY 8 IN TJSJ WAS IN USE FOR TKOFS AND LNDGS. THERE WAS A SIGNIFICANT R XWIND AT THE TIME; AND SEVERAL LNDG ACFT WERE OBSERVED WITH SUBSTANTIAL WIND CORRECTION ON FINAL AND ON LNDG. THE B757 IMMEDIATELY AHEAD OF US WAS CLRED INTO POS. I HEARD ATC TELL THE ACFT THAT THERE WAS A HVY ACFT ON AN 8 MI FINAL. THE B757 WAS CLRED FOR TKOF. ATC THEN CLRED US TO POS AND HOLD ON RWY 8. WE RAN THE TKOF CHKLIST DOWN TO 'BLEEDS/HIGH FLOW -- OFF' AND KEPT THE BLEEDS ON. WHEN ATC CLRED US FOR TKOF; I IMMEDIATELY FINISHED THE FLOW; READ BACK THE TKOF CLRNC; AND VERBALLY FINISHED THE CHKLIST. THE CAPT HAD STARTED HIS TIMER WHEN THE B757 HAD STARTED THEIR TKOF ROLL. FROM THE BEGINNING OF THEIR TKOF ROLL TO THE TIME THAT ATC CLRED US FOR TKOF WAS 55 SECONDS. AFTER APPROX 5-10 SECONDS; THE CAPT HAD NOT ADVANCED THE PWR LEVERS. JUST AS I WAS ABOUT TO PROMPT THE CAPT; ATC CANCELED OUR TKOF CLRNC AND TOLD US TO IMMEDIATELY TURN R OFF THE RWY AT INTXN S-2. I READ BACK THE CLRNC AND WE COMPLIED WITH ATC. WE WERE THEN INSTRUCTED TO DO A 180 DEG TURN AND HOLD SHORT OF RWY 8 AT INTXN S-2; AND SUBSEQUENTLY GIVEN TKOF CLRNC SEVERAL MINS LATER. AFTER DEP; WE WERE INSTRUCTED TO CALL TWR WHEN WE LANDED BECAUSE THEY 'HAD A QUESTION' FOR US. AFTER REACHING CRUISE; THE CAPT AND I DISCUSSED WHY HE HAD HELD OFF ON ADVANCING THE PWR. WITH THE R XWIND; A LARGE ACFT TAKING OFF BEFORE US (ALTHOUGH I DO NOT REMEMBER IF THE B757 WAS DESIGNATED AS 'HVY' AT THE TIME); AND THE EXITING BUMPY CONDITIONS; THE CAPT WAS CONCERNED ABOUT THE WAKE TURB/VORTEX FROM THE DEPARTING B757. IN RETROSPECT; I COMPLETELY AGREED WITH THE CAPT'S CONCERN ABOUT THE SITUATION.