Voopoo coils can last various lengths of time depending on the device, coil size, wattage, etc. The best way to look at this is by size and resistance. Voopoo coils that are smaller and meant for smaller devices will be designed for lower power output. This means that these devices will produce minimal vapour and are not able to be cranked way up in power. If you do this, it is likely you will burn out a coil much faster. Larger coils by Voopoo, which are for larger tanks, tend to last a lot longer because there is more cotton within the coil to be repeatedly heated before fully burning. When a coil is larger it will take longer to burn out.
Filling a Voopoo PnP pod is a very simple process. Simply remove the magnetic pod from the top of your device and turn it upside down. On the bottom is a steel slide lock over a rubber seal. Pull the lock back, pull the seal up and insert your bottle tip into it. Once you have filled it to the top, re-seal the pod and place it back on top of your device.
If your Voopoo Drag keeps turning off it could be for a number of reasons. The first likely reason is that your batteries are getting low and need to be charged up. The device will not work well on a low battery. The second is that your batteries are getting old and have dents in the tops of them. This will make the battery rattle around inside the device and not touch the connection properly. This will cause the device to flicker or turn on and off.
There is no particular best Voopoo device. All of Voopoo's devices are designed to fit your needs specifically. If you are looking for a smaller lower power hit then go for a pod system such as the Drag S or Voopoo Vinci. If you are looking for something a little stronger then go for a Drag kit with a dual battery set up.
The Voopoo series has a very simple system for adjusting the wattage. Simply use the up and down buttons to scroll through the various menu settings. Make sure your wattage is set to the correct rating for the coil you are using.