A vast majority of Bollywood films don't even have a script !!! Such films are mainly driven by star power and the presence of a script if any is purely incidental.
A few films that do have a legitimate script have one that is extremely weak and half baked, or which caters to a particular 'formula' that has already been done to death.which basically implies that its nothing but old wine in a new bottle.
Then there are films which are 'inspired' by Hollywood classic.
Finally, you come across a few rare gems where the script actually happens to be the primary strength of the film, and consequently its USP when it comes to selling the film.
Now to answer your question, for films falling under the first category, a script writer is not even included the film making process. In the second and third categories, the script writer's job primarily entails recycling scripts of recent blockbusters or indianising a Hollywood classic, which according to me does not require a vast amount of effort or creativity. In the forth instance however, a writer is required to invest a significant amount of time and effort in developing a truely innovative, orgininal, interesting and engaging script.a task which at times even stretches across multiple years.