When do you know when to change from TR55 to TR6 plugs (HP shot) and how many miles can you get out of them when the contour requires double plats?

Alex 00 T-Red/Tan SVTC #1964/2150 167 HP @6800 153 TQ @3000 Sold- 98 'Tour SE 24V 5-spd 149 HP @5900 148 LB FT @4300