Reed diffusers may take a little time to kick in but they last for an insanely long amount of time. On average, 100 ml of reed diffuser oil will cover you for about a month.
But which reed diffusers last the longest? This depends on two factors: the quality of the diffuser and how well you take care of the product.
Organic diffusers with stronger scents tend to last a fair bit longer. You also need to take proper care of the diffuser if you want it to go the distance. This means closely following the instructions manual of your diffuser.