Sooner or later you will complete a piece of music. Occasionally, the end point is determined in advance, for example, you have been commissioned to write a work within a certain period. More often than not, it creeps up unnoticed.
There is little published data on how long composers take to write. Probably the best guide is to look at the typical timescale that external bodies use to commission a piece of music. This ranges from about three months for small works to two years for large works. Small works contain less than 10 minutes of music, large works, over 30 minutes.
Here is an entirely subjective classification of music by length:
As a rough rule of thumb, one minute of music takes between a day and a week to write.