Bearing in mind that every performance of a script will be different dependant upon the speed of the reading and acting, I can only offer a rough guidance. The nativity service, by nature of its deliberate flexibility, includes lots of elements that are optional and can therefore be omitted or adapted, but the following individual timings give an idea of how you could adapt the play to your own circumstances using whichever elements you choose to include:
Bible readings: 6 minutes (very much dependant on the reader’s speed and emphasis)
Leader’s introductions of songs: 1 minute (these introductions are scripted in rhyme, but you may choose to add your own personal touch)
Storyteller(s), and actors (ie, basic play element): 6 minutes (This is based on each actor being on stage ready when they are introduced, so you would need to allow substantially more if each individually needs to get in placed after they are introduced. Its surprising how many minutes are added in individual 10 seconds!)
Roll call poem: 2 minutes (be aware that that this poem is different in length dependant on how many of your cast stand and say their line and with how much emphasis, speed and efficiency they do so! The poem gives everyone in the cast the opportunity to bow and respond at the relevant point so is a very special addition)
Free poem to round off the service: 1 and a half minutes
Songs are, of course, optional and your own choice so will lengthen the service as much as you choose according to what you include.
These timings will, of course, be reliant upon everything flowing in a calm, fully rehearsed way without delays to get actors in position etc. which depends very much on how you do things and can make quite some difference over the whole of the play/service.