It all starts with the script; you need to do a complete preproduction breakdown to determine the specific requirements of the project - minimal or numerous locations, small or large cast, lots of stunts/effects or minimal stunts/effects…… you get the idea.
The idea is to cram as much as you can into your budget. It comes down to compromises. You could hire Randy Thom and his team to do the audio post, about $100k - $250k per linear minute, or someone like myself who will do it for substantially less, much, MUCH less. You could retain Jeff Wexler and his team to do production sound at about $2,500 a day (including equipment fees) or a one-man-band freelance PSM for $100 to $500 a day. The same applies to every actor and crew member.
And don't be deceived by the success of "low" budget indie projects. "The Blair Witch Project" is invariably trotted out - "It was made for only $60,000!" Which is entirely true - if you ignore the almost one million dollars invested in audio post and another $250k put into cleaning up the edit and the color correction. Add in other similar things and the actual cost (less marketing, legal, etc.) is still under $1.5 million; a very small sum for "Hollywood" but an astronomical sum for most indie filmmakers.
In truth "BWP" would be substantially less expensive to post these days as there have been enormous leaps in affordable technology and the availability of talented postproduction folks to use it since "BWP" was released.
It's all about exquisite planning and attention to detail during preproduction. If you are as organized as the D-Day invasion, giving full support, "training" and briefings to your army of cast and crew, and preparing for all possible problems and having back-ups for dealing with them, your shoot will go relatively smoothly.
That's the best I can do with your 'how-long-is-a-piece-of-string question.