Pre:heating a BBQ is not rocket science and is necessary to get a BBQ up to a cookable temperature...
Now some Gas BBQ Gurus will tell you to preheat your BBQ with all the burners on high and the hood down until the temperature guage goes into the red! I really consider this unnecessary and risky as it risks catching fire and can harm the internal components if its too hot.
My rule of thumb is, when you cook a roast get the temp with the hood down up to 200c, and maintain this temp while cooking by altering the burners to high or low as you go. Going over this may result in your food being burnt on the outside and raw on the inside.
With cooking steaks or chops etc. on an open grill, I tend to preheat with the hood up so I can see whats going on and it stops the whole place smoking up! 15-20 minutes on high with the hood up gets a gas BBQ plenty hot enough believe me......
Now a charcoal or briquette BBQ is different as you need to get the coals glowing bright red before you cook anything otherwise it wont be hot enough, again about 20 to 30 minutes should be enough time depending on the number of coals / charcoal you use.
Note: If your gas bbq does not get up to temeperature dont blame the BBQ as its probably a gas supply / pressure problem.
