3 weeks or 5
It ia actually pretty hard to make a DIY for that price unless you have all the materials on hand.... buy a clear gravel tube $8 - $10, length of silicone tubing $4, tube of aquarium silicone $8, power head $13, plus shipping as those are internet prices, multiply by 2 for LFS prices.
I find the DIY mix in the Hagen unit lasts 3 weeks. Co2 levels are OK not outstanding, due more to the Whisper filter than the Hagen unit.
For a 10 gallon tank, the Hagen is fine. Foolproof and leakproof.
It is a great way to begin using CO2, as someone mentioned in another thread, "everytime I DIY I end up spending more than the thing would cost to buy".
Start with a Hagen unit on a small tank, then, one the next BIG tank, DIY. By then you'll understand most of the details.
2 x 10 g planted tank with Hagen units
29 g planted tank with DIY
59 g planted tank with pressurized CO2
58 g fish only
100 g fish only