There are two factors to consider when timing contractions. How far apart they are, and how long they are. To measure these, keep track of the start and stop times of five consecutive contractions.
Here is an example to help you understand:
In this example, the contractions are two minutes apart, since it takes two minutes from the start of one until the start of the next. The duration (how long they last) is one minute because it takes one minute from the start to the end of the contraction.
Your contraction pattern may not be that exact, which is ok. Remember your body is responding to the rise of hormone levels, not the clock. When you want to time your contractions, mark the beginning and ending times for about five, then get an average.
Here is another example for you:
In this example, averaging out the times, it seems that these contractions are pretty close to five minutes apart, and last around 45 seconds.
One last point about timing. You do not need to track every contraction of your labor. When you feel something has changed (they seem to be coming stronger or faster, or the mother seems to be working much harder) then average out about 5 contractions to see if there is any change in your pattern. Timing every one is a waste of your energy for information that has very little use.