Always strive to do your best, but take into consideration uncontrollable factors like time.
Suppose we are entering a sand castle making contest, and the lucky winner will receive a new pick-up truck. You are probably already thinking about how to build your castle. Now ask yourself how time can affect the quality of your castle. If you were given a week to build the castle, it would probably include a tower, a mote, windows, and a working drawbridge. If you were only given 30 seconds, the castle would probably resemble a square shaped ant hill. In this example, time plays a significant role in the outcome of your castle, regardless of your effort. This concept carries over to many situations in life.
When you are confronted with any task, the first question to ask yourself is how much time and effort is needed to accomplish it. If while hiking, you come across a stream, you may want to use a log to make a temporary bridge. But if this stream was on your property and used daily to get to your house, it might make more sense to construct a proper bridge.
Trying to build the Great Wall of China in a day will surely lead to disappointment, just like using chewing gum to fix a plumbing leak is a bad idea. The appropriate time and effort must be factored into every task to get adequate results.
If my boss asks when I'll have a report ready, I usually estimate the time and add another day or two. This way when I finish early, I look like a hero, and if I run into problems I can still get it to him on time.
By understanding time constraints, you can better allocate you time and paved the way to achievable success.