Been dealing with issue for 3 or 4 years. Shower (on a septic system) starts smelling bad within 5 seconds of using it. It has a P trap and a guy just installed a studor vent about 2 feet downstream of the trap...that seemed to help a little but not sure. The smell stops after the shower is off...after 10 minutes or so. Please help

a studor vent only allows air to enter the system.
you have a positive pressure building up when you use the shower so a studor is useless.
temporarily remove the studor to see if the smell comes out that open pipe rather than backing up into the shower.
so if removing the studor allows the smell out then you need a proper vent pipe to let the foul air go to the atmosphere.
a cigarette or a smoldering piece of cardboard held over the open pipe will tell you what is happening.
may have to experiment a couple of times with water not running and then with water running.

Thx for your help. I have a vent through the roof that i could tie it into. It's about 12 - 15 feet from it horizontally. Will that work?

do the testing i suggested before making any more changes.
i could be wrong in my assessment of the situation. (i'm not infallible).
to answer you question, the vent could be run the 12 feet but there are certain criteria that must be met.
the main one is the vent can tie in to an existing vent but it can not tie in below the flood rim of any fixture being served by that vent.
so its not all that technical to understand but take one step at a time.
if running the vent is needed then a picture or two will help.

when's the last time you have the septic serviced?

Since you have a septic system, do you also have well water? What happens if you just run cold water in the shower? Some well water reacts with the anode rod in a water heater causing a sulfur type smell, cold water would not have the odor.

Good catch, Sharp1. I didn't even think about well water. Yes, an anode rod can cause very foul odors with certain well water.