Cant help you with duration but when I tried to program my G5 originally I had your problem of activation whenever anyone spoke. I realized I was setting up alerting completely wrong! Make sure you setup your D3 protocol parameter as a P25 Trunking (and under that protocol, I am using 100ms short / 200ms long / 1000 ms gap, and then set your talk group in D5 (alert mode tone, pick an alert tone, program the tone group frequencies (for me its 2 tone phase 1). In D6, click on your zone / function knob setting, then under binding feature, select "sub group feature" and under that, auto reset (or whatever you want), and then scroll down to bottom and select "sub group setting" and add the talk group, and then "enable". If you want to monitor the channel also select "alternatives - enable) otherwise G5 will stay silent until it alerts. I have 2 knob positions setup - one to stay quiet (alternatives - enable not selected) and another to monitor channel (alternatives - enable selected).
Not sure if this helps. When I first set this up, I tried to select the protocol under D3 as "2 tone" and I was going down the wrong fox hole!!!
If this does not help I will leave it for others to respond. Good luck!