Much depends on many variables:
-proper, long cure vs. short cure
-washing/bathing in hard water vs. soft water
-big, hairy people vs. children or smaller people with little hair
-direct, on-the-body lathering vs. using a facial cloth or nylon pouf
-leaving the soap in the direct line of shower spray vs. keeping the soap out of the direct line of spray
-storing the wet soap on a surface with no drainage vs. storing it on a well-drained soap dish so it can completely dry out between uses
-formulating the soap recipe to be heavy on the soluble fat side, such as high coconut oil soaps vs. being heavy on the saturated fat side
-small bars v. big bars
-short showers vs. long showers
-showering once a day vs. twice (or more) per day
It usually takes me about 1 month to go through a 5 oz. bar, but I'm petite (5'1") and don't have a lot of body hair or body mass. Plus my bars are cured for at least 4 to 8 weeks before using, I keep them out the line of shower spray, and I store them high and dry on well-draining soap dishes.
My hubby and son, on the other hand. share a bar and usually go through one of my 5oz bars about every 8 to 10 days. They are both much taller and hairier than me, son takes two showers per day (before and after work), hubby takes 2 showers per day on the weekend (after yard-work, etc..) and in spite of my constant encouragement, they like to keep their bar in the built-in soap shelf in the shower wall that gets hit with a lot of shower spray (albeit on top of a well-drained soap dish, but still). They say it's a more convenient place for them. I keep mine high and dry up on the shelf of the shower door frame, which is the perfect width for holding my bars and soap dishes.