The first advice we'd like to offer is that you get a magnifier, such as a jeweller's loupe or a microscope. A good beginner's loupe should be able to magnify up to thirty times. If you're thinking about getting more serious, we recommend a professional magnifier with a magnification of about 100x.
Now that you have a loupe, it will be much easier to determine when your plants are ready for harvest. Check the tiny, sticky resin glands, the so-called trichomes, that should be covering the buds. Trichomes progress from clear, to cloudy to amber as they ripen. Most growers will harvest when the largest number of trichomes has turned cloudy, with just a few amber ones, and a couple more clear ones. But it's easy to get it wrong. Harvesting too early will have a negative effect on your yields and if you're really early, will also affect your bud's potency. Harvesting too late can cause THC deterioration leading to a bad stone, and also carries a greater risk of mould.
Since trichomes do not ripen at the exact same rate, you will have to accept that some of them will have gone amber already while others are still in the unripe clear stages. Just try to time your harvest so that most of them are cloudy and amber.
To be able to determine when to harvest, it helps if you know a little about the processes that occur during the ripening. The levels of some cannabinoids, the psychoactive ingredients of cannabis, will vary as the trichomes ripen. THC, the main psychoactive ingredients, is at its maximum strength when trichomes have turned cloudy. Clear glands are immature. Once the glands go amber, the THC will start to deteriorate, creating CBN. This will makes for a duller, lethargic stone. Therefore, harvesting a bit earlier generally leads to a more energising high. Still, if you're growing an Indica (body stone) , don't expect to get a Sativa effect (head trip) just because you harvested early - that would be a waste of a good crop! First determine what kind of effect you're looking for, buy the appropriate strain and harvest when ripe. Bear in mind that harvesting too early is not a good idea either - the THC levels will not be maxed out yet, and your bud won't be as potent.
In the end, the ideal time to harvest remains, at least to some extent, a matter of personal preference. Different strains will not ripen at the same rate, either. Professional growers usually have a pretty clear idea of when to cut their plants to get the best results. Inexperienced growers cannot really go wrong with the 'mostly cloudy with amber' rule until they learn to determine the best harvesting time for themselves.
Another helpful hint is to check the pistils of the plant: the hairy little tendrils that are often orangey, reddish or brown in colour when ripe (unripe ones are white). When the pistils start receding into the bud, that's when most plants will be ripe for the picking. Don't fret if some of the leaves on the plant start dying or yellowing just before harvest: it is natural that they should do so, since the flowering signifies the end of the plant's life cycle.
A nice little experiment is to harvest in stages, labelling and storing the different buds in separate containers. A proper scientific test would have to be double-blind (i.e. get someone else to hand you unidentified samples to compare; neither you nor your helper should know which is which) but even a normal taste test might help you decide on the ideal harvesting time for that particular strain.
Lastly, a quick note on decreasing the flowering period. Please be aware that several factors may influence the amount of time your plant needs to reach perfect harvesting ripeness. Light leaking is one of the most common ones: make sure the dark period is DARK or your plants will take longer to mature, and may even turn hermaphrodite. Also, take care to neither over- nor under-fertilise your flowering plants. Happy, healthy plants will be harvest-ready sooner than stressed ones.