Hello Andre,
in FreePBX You can custom the Asterisk Dial command. Go to Settings -> Advanced settings. Look for “Asterisk Dial Options” and “Asterisk Outbound Trunk Dial Options”
The parameter You need is “LIMIT”: https://wiki.asterisk.org/wiki/display/AST/Asterisk+11+Application_Dial
For example, to limit the call duration to one hour and warning the caller at 50 minutes every minute that the call is near to be limited (hanged up) use: L(3600:3000:60)