Scripts are generally 100 to 120 pages. Each page, on average, equates to 1 minute of screen time. Of course some action-filled pages may take 5 minutes a piece while some pages loaded with dialogue only 20 seconds but it all evens out.
One of the first things that producers check when reading the script is the length. If it is under 100 pages then it appears that the scriptwriter doesn’t have enough material to tell a feature length story. Go the other way, over 120 pages, and the script is automatically thought of as cumbersome. Once you have a solid reputation as a scriptwriter you can get away with going over 120 pages but you should stick to these guidelines if this is your first spec script.
The easiest way of keeping to this unwritten rule is to break your story into three acts. In a 120 page script the first act would take up one quarter (30 pages) of the script. Act II takes up half (60 pages) of your script. Act III takes the final quarter (30 pages) of your script.
For more on how to structure each individual act, visit the pages below.