Ansel Adams once said that "Sometimes I arrive just when God is ready to have someone click the shutter." But just what time is that, and how exactly are you supposed to know?
There is one simple answer of course, so long as you don't dig too far below the surface of that answer. You take the picture when you like what you see through the viewfinder. With wildlife, you want to catch them showing some form of indicative behavior. You want the moment when the flock of birds take off. You want the moment when the rutting elk has his neck stretched out bugling, not perhaps the time he's got his head down foraging for food. But no matter how long you hang around waiting for the most spectacular moment imaginable, that moment is sure to come just after you start to pack things up to call it quits for the evening. For general landscape images, you want to shoot at the peak of the golden light "magic hour" near dawn and dusk. But even stipulating this, you never really know when will be best. The scene may look gorgeous, only to look even more so ten minutes later. Or perhaps not. With wildflowers, you want the moment the wind is not blowing, a condition not easy to encounter some mornings. Just when you go to snap the shutter, a gust kicks up and blurs your shot. With shutter speeds lasting upwards of several seconds, almost anything can happen. The truth is, catching the perfect moment to click the shutter is harder than it first seems.
Here's another quote for you: "Time is nature's way of keeping everything from happening all at once." This one is variously credited to American theoretical physicist John Archibald Wheeler, or perhaps Albert Einstein, or even comedy film director Woody Allen. Its roots seem to go back further than any of these possibilities though, but the point is the same regardless of who first had the wit to say it. Every moment is unique and happens only once. There is an inherent orderliness to time that we can't circumvent. Each press of the shutter release creates an image at that precise moment. If you want an image taken at a later moment, you have to take an image at that moment. There's no going back in time either to fire off a shot at a moment you missed. Some people ponder whether it could be possible to go back in time and kill Hitler before his rise to power. Photographers ponder about the shot they were close to getting that morning but just barely missed out on. Both sorts of idle speculation can have about the same chances of impacting real world events.
And this creates an interesting dilemma. The only picture you can actually take is of the view available to you right now. If the light looks great, press the shutter release. Ten minutes later it may look even better and you can always take another shot then. But if ten minutes later the magic light has started to fade or a gentle breeze starts blowing and ruins the mountain lake's mirror surface, you're going to be glad you took that earlier shot. You did take that earlier shot, now didn't you?
Some people wait for perfection and thereby miss the whole shooting match. Things may improve or get worse, but perfection never really comes. Thankfully, with digital photography, you're not apt to run out of film or anything. And if the memory cards you have with you do all get filled with attempts to capture perfection, you can always delete some of your less successful shots to make room for new ones.
While you have that perfect shot framed in your viewfinder though, with the camera locked down on your tripod, you could easily be missing other possible shots. While you wait for the light to reach its peak, time is passing. How long do you wait? If you wait long enough, you might increase your chances of getting a better shot, but at the same time you might also be missing other good shots. If instead you switch lenses and recompose for some other shot, you might miss getting great images with even better light of your first subject.
See, timing can be quite a dilemma once you go beyond simplistic answers, with all due deference to Ansel Adam's skills as a photographer or course.
If budget and sheer weight weren't obstacles I'd carry additional camera bodies, lenses and tripods with me so I would be ready to do more than one thing at once. Not that I could actually shoot multiple cameras at the same time, but with enough equipment I could have one setup locked down and ready to go for one subject, while I had a different camera pointed the other direction for something else. At sunrise, it's not just the eastern horizon that bears keeping an eye on. But there are limits, and my back tells me my weight limit isn't quite as high as my dreams wish I were capable of. Perhaps I need a Sherpa, but I digress.
Maybe it's just me, but I find myself thinking about this sort of thing, sometimes while I'm on location and discover that I've missed great images as a result of making choices necessary for shooting other images, and sometimes just around the house while I think about how not to miss such great images next time I'm out shooting.
Ultimately, the only time we have is now. We can remember yesterday and plan for tomorrow, but when it comes down to it, you have to decide in the present tense whether to press the shutter release or not. All you can do is use your best judgement as to which of the many possible shots to focus on in any given situation. I would say to also keep your fingers crossed for good luck, but you need them to handle your camera and press the shutter release to take pictures in the first place.
The reality is that there are no easy answers. Timing is inherently tricky. My advice is to carry a lot of memory cards with you, and perhaps a tad more gear if you feel up to it. And don't get to frustrated if you do miss some good shots. Even if you don't capture them all, hopefully you at least were there to witness them. It can be awfully beautiful out there some times. Now that's timing.