The amount of time needed to dry seeds varies a lot by species. The best method to determine if seeds are dry enough for storage is to measure the relative humidity of the seeds, especially the first time you work with a new species of seeds.
Hygrometers are devices that measure the relative humidity (%RH) in the air. If you have a small hygrometer sensor (like a Blue Maestro Tempo Disc or similar), you can place it inside a container mostly full of seeds, then close the container with an airtight seal, and the hygrometer will measure the equilibrium RH (eRH) between the seeds and the air inside. This eRH can then be used as a proxy to estimate the moisture content of your seeds, without using methods that destroy seeds.
You may need to wait several minutes or up to a couple of hours for the sensor readings to equilibrate (in other words, to stop increasing or decreasing), which indicates that it has reached the RH that is in equilibrium with both the seeds and the air in the container. This is why it is important to minimize the amount of air inside the container, to reach equilibrium sooner, and to get a measurement closer to the true RH of the seeds. However, even if the jar with the hygrometer is full of seeds, there are still air spaces in between the seeds.
If the air in your seed bank is dry, the eRH measured by the hygrometer will be fairly close to the true RH of the seeds. If the air in your seed bank is more humid than the location where you are drying your seeds (such as a dry cabinet or dry box), then you may need to allow more time for equilibration, and you can assume that the true RH of the seeds is somewhere between the %RH measured inside the container and the %RH of the air in your seed bank.
For example:
- Your seeds have been drying for 3 weeks in a dry cabinet set to 40% RH
- The air inside your seed bank is at 50% RH
- The hygrometer inside a container with seeds and a little bit of air space, after equilibrating for an hour and reaching a stable measurement, reads 40%
- Then, the true %RH of your seeds may be closer to 45% (depending on how much air is inside the container), so you should dry the seeds a little longer, then measure eRH again
Remember, seed drying is not an exact science, and all measurements of seed RH will be estimates. It is ok to dry seeds to +/- 3%RH relative to your target RH, and even imperfect or partial drying of seeds before storage will usually extend their storage time significantly, compared to storing seeds without drying.