Our 1991 P42 Isomat mast is likely a model NG-70 according to Rigrite, which is 50', 10" long. Given that the double Dutchman lines extend from the end of the boom to within about five feet of the mast head, where I connected the two via a bowline knot rather than a shackle, makes them about 60 feet long. These lines are separate from the actual topic lift line. The topping lift line needs to be long enough to extend through the mast (51'), plus six feet to connect to the double Dutchman lines, and another 51' in order to drop the Dutchman lines down to boom level for any kind of adjustment or repair. Then another twenty feet to run from the mast base through the organizers, sheet stoppers and into the cockpit for winch work.
The above assumes that the OP's 1997 P42 came with a Dutchman flaking system. If not, then the length needs to take that into consideration. Either way, it seems to me that 120 feet should be long enough, in my estimation. Perhaps the OP could shed a few more specifics about how his setup is configured.
If I can remember (difficult these days), I can measure ours when I travel to the boat next weekend. That way I can be more specific about the length on our boat. In the meantime perhaps the OP can give a few more details about his arrangement.