Synthetics like IBA and NAA, probably indefinitely. Some say they must be kept dry. Some natural auxins may be very short lived outside the plant.

I did a little research since I also have a bottle of Schultz "take-root" rooting hormone (now marketed under the "Garden Safe" brand name) that is at least 5 years old. The main ingredient is indole-3-butyric acid (IBA) which is one of two main chemicals these rooting compounds use (the other being napthalenacetic acid or NAA). Based on manufacturers data sheets, there seems to be some discrepancy as to how long they will remain effective. For IBA, I've found ranges from 6 months to 3 years, provided it is kept dry and out of direct light. However, the MSDS sheets all rate it as a stable compound, which seems to mean that it will last for quite a while as long as it is stored properly (cool, dark and dry) and out of reach of reactive compounds that would cause it to degrade.

For NAA.. it seems like 3 years is the common figure given for a shelf life, but again, it is listed as a stable compound, so it may last longer if stored properly. Both compounds break down quite rapidly in the presence of UV light (NAA is stated to last only about 10 hours when exposed) and in the presence of micro-organisms such as those found in water and soil.

I've been using the Schultz product for years and have had great success.. even though it's a bit old. Although, I must admit that I also use saliva in conjunction with the rooting hormone on all my cuttings except for the poisonous ones, so that may make a difference. Unless you are working in a commercial environment producing bazillions of plants, I find it hard to go through a bottle of the stuff fast enough.. a 2oz bottle will be enough for thousands of cuttings, so it tends to last quite a while.

Footnote: After doing this little bit of research, I stopped by K-Mart on my way home the other day and picked up a fresh new bottle! $6 every 5 years or so isn't going to break the bank, and it can't hurt to get some fresh stuff every now and then. Maybe I'll try both on my next batch of cuttings and see if there is any difference in rooting between the old and new.

Cheers,
Brad