I'm presuming this fog is on internal lens elements.

Does the haze inside the lens actually cause any deterioration of the image? If it does, I'd opt for a good professional technician to disassemble, clean, lube and adjust the lens right now.

If it's not an image quality issue, I'd just keep on using the lens as is until it absolutely had to be cleaned.

Of course, if it's fungus you had best go ahead and get it taken care of immediately.