It doesn't actually get fully ripe until 16 weeks but mold does become an issue the longer it goes. Some strains can take it, some not. This Ice Cream strain is more mold prone than some others. There was the beginnings of bud rot on one pheno so I pretty much had to take the top buds off even though it was only 12 weeks in. The lower buds are still fine though and need more filling out.
Calyxes don't really expand fully until about week 14, I've found, and they still fill in more right up to 16, which is when they pretty much have to come down because leaves have yellowed out so much and to keep the cycle time reasonable. But even then the trichs are not all amber, so that tells you that trichs are not a reliable indication of ripeness, at all. You just flower them for 15-16 weeks, no matter what they look like. They do keep gaining weight, though at a lower rate when they start taking up less water in the last few weeks. Why harvest when you can still get more weight by leaving them longer, and they keep getting more potent and better flavor? You just use 3 chambers instead of 2 and give each 5 weeks, or a little more if you want 16 week weed.