The value of a musical note only indicates its relative duration. Sometimes it is not enough. That is where you need to know the precise tempo of a composition. Use it to calculate how long a musical note with the given value lasts.
Additionally, there are modifiers of duration. The dotted note lasts one and a half times longer. The triplet note is 2/3 of a standard one.
The time signature and the number of measures allow you to calculate the length of the entire composition.