Things to consider
“Do I want synthetic or orchestral/acoustic music?”
The style and instrumentation of music often affects the development time of an OST. For example; a synthetic piece of music similar to what is used in my original soundtrack for Mount and Shoot, while still of high quality, would take less time to write and produce than more orchestral compositions in my RPG Fusions Music Pack.
“Do I want static or adaptive music?”
Some video games benefit from having a single loopable track in various locations, whereas others require a more adaptive approach to composition, meaning that there are multiple layers to the music that could be triggered by moving to a new location, engaging with an enemy, or even changing from day to night. It is entirely up to you how you want your music to behave in-game, however the more adaptive the soundtrack, the more time it will take to complete.