How far back should the gear levers be back on an old racer bicycle mine goes back parralel with the frames their fixed on

It all depends on the number of cogs on the rear wheel and how ell adjusted/setup your system is. Do you have play in t he lever before it shifts from the smallest cog to the next cog? When the gear lever is pushed all the way forward is the cable loose? If yes to those questions then the cable needs to be snugged up. Slightly loosen the bolt/nut holding the cable to the rear derailleur and then pull the cable until it's snug. Tighten the bolt/nut. You'll probably find that the gear lever doesn't need to move as far to change to the biggest cog on the rear wheel.

Some people change the number of their rear cogs but not the gear lever and sometimes that causes the gear lever to be pulled back further than was intended in the original setup. Also, some gear lever have a larger diameter area where the cable wraps around. If that gear lever is replaced with one that has a smaller diameter then the smaller diameter gear lever has to move further to move the same amount of cable as the original one. Either of those scenarios can cause the gear lever to end up being parallel to the downtube when in the biggest cog on the rear wheel.

Cheers

It will vary by shifter model and the diameter of the cable drum as well as the components required cable movement. As stated, adjustment procedures or preferences as well.