Over time, some types of colloidal gold may show signs of the gold falling out. You may see black residue on the bottom of the bottle. When this happens, the colloidal gold has become weaker. But really there is no 'shelf life' per se for gold.
If a colloid is stable, it means that the particles will not change in size, or chemical makeup.
When you try to evaporate gold or any other colloid, you are increasing the ppm of the colloid, and you are decreasing the volume. This forces the nanoparticles closer together, and if they are forced close enough together they will combine to make larger particles. This happens much faster if there is no stabilizer in the colloid. In the end, you will still have all of the gold, but the particles will be larger, and may not reconstitute if you add back the water.
There is one other issue with 'shelf life', and that is sterility. Colloidal gold has no antibacterial properties, so it is possible for it to grow molds and bacteria.