Photograph opportunities often are not available for an infinite amount of time. In fact, most are very, very finite and there are many times you have just one shot at making a photograph. How can you insure you will always give yourself the best chance at making a great photograph even if you have just one shot at it? By being able to change the settings on your DSLR very quickly.
It depends on your particular DSLR and what dedicated buttons you have available. No matter what DSLR you have, even if you have an entry level one, you should be able to change all five necessary settings for making a well exposed and sharp shot in ten seconds or less. If you have a fully functional DSLR, i.e. two dials for changing settings, a top LCD display and dedicated buttons for all five things, then your goal should be five seconds or less. Very rarely are all five things needed to be changed, but you should practice changing all five at home so that when you are out shooting you will be prepared.
Of course you have to know within fractions of seconds what to change your aperture to, or your shutter speed to, etc. That knowledge combined with knowing your DSLR camera body with your eyes closed (seriously, if you have dedicated buttons you should be able to operate them eyes closed) results in giving yourself the best chance every time a sudden photograph opportunity comes up.
I offer 1-on-1 DSLR Photography Lessons that can help you learn both how to use your camera quickly and what to change the necessary settings to. Reserve a lesson today!