1. Dark Regen + Glyphed Healthstone macro: a very large 10s HoT on a 2m cooldown.
2. Ember Tap (Destro only!): a spec-specific instant medium heal at the cost of their main damage resource.
3. Drain Life: heals for about 8000 per second, less debuffs. Inconsequential.
4. Glyph of Siphon Life: heals for less than Drain Life. Meaningless.
So if you're facing a Demo or Afflic, all you have to worry about is the purple swirly heal every 2m. Destro can hold out longer, but every Ember Tap is a Chaos Bolt they can't cast.
Here's the thing with Warlocks. They are very hard to kill when they're chaining their defensive cooldowns. When those cooldowns are gone, they're about as durable as tissue paper. The trick to killing a Warlock in PvP is to just keep the pressure up. If you chain your CC till those cooldowns are gone and don't die or let them shake you off, they will squash quickly.