The length of time it takes for methylene blue to fade depends on various factors such as the concentration used, the type of material that was stained and the environmental conditions. Generally speaking, methylene blue will start to lighten in color within a few minutes after application, but can take up to 24 hours before complete fading occurs. The most common concentrations used are 1% or 2%, so if you have a higher strength solution then expect it to take longer for any remaining dye to completely disappear. If you’re using methylene blue on fabrics or other porous materials like wood, paper or stone, then expect some residual staining which may not fully fade over time and could become permanent without proper treatment. Furthermore, sunlight exposure can also cause quicker fading while humid environments tend to slow down the process significantly. Ultimately though, with proper handling and good storage practices you should be able to keep your item looking its best for many years!