Depends on the target of interest. Most samples are relatively stable in buffered GA/FA fix at 4C, and can be used for IEM studies after weeks. That is under the condition that the antibodies are working with the chosen fixative solution. GA, especially at higher concentrations will dramatically reduce antigenicity for most antibodies.

As Martin wrote above, the glutaraldehyde concentration is the issue here, not the storage. Many antigens become impossible to label above 0.1% glutaraldehyde. Samples can be stored in 4% formaldehyde buffered in PBS at 4 °C almost indefinitely (as long as you top off the fixative as it evaporates over time). I have successfully labeled GFP in drosophila tissues that were stored for monthshttps://doi.org/10.7554/eLife.46421