My pair spawned for me the first time in under a year, and I guess they were about 4 months old when I got them. So definately on the low end of the range. Why? I have no idea. But good food and lots of it from the time I got them probably helped.
I took my juvenile G. oceanops a few months, the yellow watchman gobies 30 days, my scooter blennies 3 days, and the pistol shrimp pair arrived with eggs!