As several have mentioned, the length of the high depends on a lot of factors including how recently/what you've eaten, how hydrated/stressed/etc you are, along with many other factors. Genetics sets the foundation for what a strain will do and how long or intense the high will last. Growing factors affect that too. Harvest decisions do affect how 'up or down' the effects might be, but you're not really going to take an extremely sedative strain and make it energetic by harvesting early or vice versa. It's kind of like cooking salmon. It's going to be salmon regardless of how long you cook it. Some people like it a little bit more rare and others like it more cooked through and crispy on the outside. No matter how long you cook it, it's not going to be a steak.
Where the color of the trichomes comes in is helping to 'see' what cannabinoids are there. As the trichomes turn amber, that shows that some of the THC is changing (through degradation but that's a negative term and this change isn't good or bad) to CBN which is what is mainly responsible for the couchlock feeling as CBN is one of the 'sleepy' cannabinoids. There is a sweet spot that the breeder has aimed for and as a rule of thumb, it's when about 10-30% of the trichs have turned amber. Consider that a medium rare filet. Some prefer to harvest before that and enjoy a rare steak while others prefer to harvest after that and prefer a more medium well steak.
Not sure if any of that helps, but it has made me hungry!