I'm assuming you're talking about a 510 cartomizer with a DCT tank? If so, hopefully some of these answers will clear things up...
1. 5 or 6ml tank for heavy vapors - will last 1 week even for a heavy vapor (such as myself).
2. Glass cylinders will last the longest because they can withstand darker ejuices and 'spicy' flavors. However, I've been using cheap
$7 Boge F16 tanks and they've worked just fine, none have leaked so far.
3. Do NOT toss the tank unless the cylinder itself is leaking. You can keep refilling it for a long time, if not forever. The tank itself is made of 3 parts - top cover, cylinder, bottom cover.
http://ecigmodshop.com.au/images/uploads/Cartomizers/BogeF16-Structure.png
Replace any of these parts as needed. Think of the tank as the bottle for bottled water - no need to throw it away unless it's obviously overused, just refill it with water.
4. If the cartomizer is dead, replace it. In the entire tank setup, the cartomizer is really the only part you need to replace. Typically, a carto lasts 2 weeks if vaped on a daily basis.
Hope this helps!