The Production Department and the Director need to know how long each scene will run for to schedule and budget the project. If a script times slightly longer than originally thought it’s generally no problem. If it’s too long, then some scenes need to be shortened or even cut to not waste money and time shooting action that won’t make it in the finished project. If it’s too short, then the Writers need to add in extra actions or scenes to be able to cover the screen time needed. This task is pretty important specially in TV where networks are very specific with their time slots.
The general rule is that one page of script formatted as per industry standards (Final Draft, Celtx, Montage, Movie Draft, etc) equals one minute of screen time. However this is not always the case as dialogue usually runs for a bit longer and some actions described in just a few sentences may be covered extensively (general rule for chase scenes or fights). It’s best to check with the Directors on their chosen style for the shoot – ask about pace and those tricky action sequences.
I always do a few loud readings of the script and time myself with a stopwatch. I write the times for each scene in a table. If there’s a read-through in preproduction I time that as well. Read-throughs will give you a general feel for the performance but they are not the final performance so they run a lot faster than when they are shot.
Timing Scripts might seem scary at first, but don’t despair, the more you do it the better you’ll get!