How long the cartos will last varies by user. Some use them a few days and toss them. Others can make them last a month if you baby them. Try to always keep them topped off (adding a few drops of juice as soon as it dries out just a little, not a problem with the tank cartos). Keeping them good and slushy seems to help them last longer. By all means DO NOT let them dry out to the point where you get a dry burnt hit. The carto will be toast after a dry hit. You can also flush them out with water and let them dry well to extend the life. Here is one of the many videos that will help with cleaning cartos:
DIY is great. Do go to the DIY forums and READ READ READ though. It can be dangerous playing with nic- make sure you know how to handle it. VG can be found in the pharmacy section at Wal Mart or any other pharmacy. It is usually on the bottom shelf in the first aid section. PG can be found at tractor supply places, usually in large quantities though. Myfreedomsmokes has good prices and good quality nic liquild, as well as all other supplies you will need to get started (including flavorings).
Best Electronic Cigarettes and E-Liquid - MyFreedomSmokes.com I recommend not going above 48mg/ml, or 60 at the very most, for starting out. Again, this stuff can be dangerous to play with and those levels will keep you safer. They also sell the pg and vg if you just want to get it all in one place, since you will be paying shipping anyway. Start simple- one flavor or two flavor combos. Complex recipes are usually more than just throwing together some stuff that sounds good. Getting the right strengths can be an art. I tend to like the one or two flavor juices anyway, so it works well for me.
The initial investment can be a little high for DIY, but you can also save a TON of money brewing your own juice. With that initial investment you will have enough to make months worth of juice.