Years ago, a study by APQC found the most successful teams invest 20+% of their total new-product work in the front-end… while most teams spend 10% or less here. Roughly speaking, a Market Case requires about 3 person-months of effort, while a full Business Case needs about 6 person-months. So if you “do the math” for 20% in the front-end, you should require a Market Case for projects needing 1 person-year of development, and a Business Case for those needing 2+ years.
More in article, Business Case Excellence: The 12 Key Components