Maybe a simple question, but how long do silica bags stay good for. If I keep them with my mics to protect from humidity, do I need to change them occasionally or are they good forever...?
Silica gel's high surface area (around 800 m²/g) allows it to absorb water readily, making it useful as a desiccant (drying agent). Once saturated with water, the gel can be regenerated by heating it to 120 °C (250 °F) for two hours. Some types of silica gel will "pop" when exposed to enough water.
If you can get the gel out of the bag, and if they are hard little beads, what you could do to reuse them is put them on a fry pan, without the oil of cause, and heat it up until it changes color. You'll have to put them back into the bag quickly before they start reabsorbing moisture again.
I've generally given up on trying to use them because they are too troublesome to reuse. Lazy me. Ha