A global is 1 second. Theoretically you can Mut+Envenom 3 times in a 6-second window. Realistically, you will run out of Energy if you try that, and 2 Envenoms is the practical limit.
No offense, but if you don't know what the GCD is, and you don't know how many combo points the abilities generate, and you don't have the shoulders, maybe don't call my opener "very bad", and maybe don't give advice to people on the Rogue forum in general.