When to use Life Tap during a fight is actually kind of tricky. Life Tap eats the GCD, which eats into rotations, which then cuts down DPS. This is why it's almost never a good idea to Life Tap in succession -- even with the 4-piece bonus of the Plagueheart set. A few Warlocks argue that the first Life Tap will benefit the second one, which is true, but that also translates to a 3 second downtime. The only good time to Life Tap is just before you run out of mana so that your next cast won't result in an Out of Mana error message and when you move during a fight and no instant cast DoTs need to be refreshed.
Otherwise, casting Life Tap needs to be at a minimum, except when you have the 4-piece bonus. In that scenario, trying to keep a 100% uptime on the +Spirit buff is ideal, but probably won't happen. In a case where all instant cast DoTs are ticking, however, it's a good spot to insert a Life Tap just before casting a Shadow Bolt. Just don't do it so often that you find yourself Life Tapping when you have more mana than you need. The ideal is ending a boss fight at exactly 0 mana. Of course, Elleiras makes an excellent point, as well, so don't waste those sweet HoTs. When you see a swirly green thing around you (the good kind), Life Tap with gusto.