The worst problem most composers suffer from is the tight schedule on their projects. Many composers sleep not more than 5 hours per night when there is crunch time which of course doesn’t allow to write the most creative music.
The main problem often lies in the fact, that originally the time frame for the music might be set quite comfortably but due to the natural delay of most departments before the music (as music is quite at the end of the production chain) but the release date not being changed that time window shrinks often to nerve-wrecking few days or weeks.
To get a proper understanding of what is adequate time frame for your composer, you need to know his/her working speed.
Most composers get to comfortably write a mean of 2 minutes of music per day, scores with few instruments and lots of relaxed music can be written faster, busy action or comedy sequences might get along slower so there might indeed be a span between 20 seconds to 10 minutes a day (which could also happen on the same movie, depending on the scene that is being scored). However the mean of two minutes per day is a good standard. So let’s say you have a movie that needs 60 minutes of music. This would mean that your composer would need approximately 30 full working days on it. On a 6-day week this would mean 5 weeks. However if you also want demos, your composer will be slowed down needing to produce them, which might end up in a mean as low as one minute of music each day. Unless you offer enough budget for him to outsource this job (or outsource the writing of the score sheet, depending on where he feels more comfortable) you might need to be prepared for this speed. Another slowdown will be rewrites which will happen on most projects, so depending on how picky you are, you might want to add another 1 to 2 weeks as buffer for that. Additionally to the writing time you have to plan in time for him/her to actually come up with the material (like themes, motifs) and the overal language of the music (which instruments, which special musical devices to make it original and unique). A comfortable frame for that might be two to three weeks. In this time he/she can also demo themes for you and do a few revisions until you agree on the material and tone. Another time factor to be added is what happens after the composition is finished. The score sheets need to be copied and prepared for the musicians, the musicians/orchestra has to be recorded and mixed. On a 60 minute score, copying will take about 2-5 days, the recording will take probably three days and the mixing another three days. With a bit of buffer time in between (travel time for the composer) you end up with additional 2-2.5 weeks.
So at the scenario where your composer has the chance to outsource the demo production or score sheet writing and can actually compose two minutes a day, with the usual amount of rewrites and the regular time it takes to prepare, record, mix you would end up on a production frame for the music of your 60 minute score of 12 weeks or 3 months. This is a comfortable time frame and can be shortened if you make more budget available. The composer might be able to outsource more tasks like orchestration, might be able to book a huge copying company which can copy the parts in 1 night etc to shrink down that time frame. Of course, he/she can also work through nights and raise the composing speed to 3 to 4 or even more minutes per day but it will make the quality suffer. There’s an old saying between film composers which goes “Do you want it good or do you want it now?”
Keep these figures in mind when you plan your post production.