How much diarrhea or for how long would a person have to have complete watery diarrhea for electrolyte imbalance to occur? How to replenish?

1 day, Gatorade. You can get dehydrated within a day of having severe diarrhea with multiple watery bowel movements. It is very important to stay hydrated as well as replace electrolytes. The best thing to replace electrolytes would be a sports drink such as gatorade. It is likely that when you are younger, your parents may have given use pedialyte which does the same thing.
It depends. For an otherwise healthy adult male it would take days if you did not eat or drink during that time. Always hydrate with water (not caffeine) and eat. If the diarrhea is uncontrolled and you are too weak to help yourself dial 911 for help. Call your doctor to get seen incase you have giardiasis, salmonella, shigella, c diff or any other bacterial diarrhea that requires prescription meds.