I didn't say all the time, but the issue is that mage can't get range on me, given we both have our cds. Too many rogues do dumb things like open up with cheapshot. Rupture + GS - thats 1000-1300hp out of the way already. Wait to see if he pops blink or FN first, Immediately vanish after he has made his decision - you don't want a FN destealthing you, don't waste any time and get immediately on that mage before blink cycle.
I find that with improved garrote + GS, it's actually no advisable to go for ambush builds on a mage, (but perhaps not if you have imp Ambush). For your 60 energy you are getting a 25% chance at 1200 damage, or 75% chance of 600, compared to a 100% chance of getting a 600 DOT on your target (around 750 if hemo has been applied), for 30 energy.
At this stage he still has his heat wave and FN, and you have 2 combo points, because blink has been burned, stealth up, and you have a 15 sec window before next blink. IOA on FN, sprint on heatwave or if he gets any distance, keep sprint up, double if need be, if you can't take him down in the blink cycle, then you should have your second vanish. There is absolutely nothing even an amazingly well played mage can do to stop a skilled sub rogue. The rogue has a large margin for error, where the mage has practically none.