That's what I'm doing right now. Takes freakin' forever by yourself - but makes the movie come alive.
At the "Hollywood" level, depending upon the type of film, it's about 2 to 10 man-hours per linear minute of film for each audio post category - Dialog, Foley, Sound FX, Mix.
So for a 100 minute film that is between 800 and 4,000 man-hours of work. A dialog driven drama would be at the low end, an action film at the high end. Indie projects tend to spend less time, but mostly because they have such small budgets.
I spent about 750 hours on my most recent feature job which was just around 95 minutes (that doesn't include archiving, logs and all the other donkey work of audio post). As strange as it may seem, if I had had a much larger budget it would have been fewer hours as an experienced Foley team could have done the Foley work faster than me doing it all myself. But, of course, if I had had the budget I would have spent a lot more on field recording, etc. which would have taken more time in the field, setting up the sound library and editing.
As an aside, sound designers tend to have specialties although they most definitely try not to get pigeon-holed. Ben Burtt is a go-to guy for sci-fi, Randy Thom does a lot of animation films, Charles Maynes is a weapons recording specialist. Rob Nokes is called on a lot for ice, snow, etc. plus he does a lot of family friendly films.