I’m going to be frustratingly un-quantitative in my answer.
To me, “significant interest” is a very personal choice for an entrepreneur because it’s inherently tied with what the entrepreneur perceives as valuable.
For example, let’s say I’m a scientist interested in curing a disease. Objectively, I’d probably want to tackle the biggest disease possible, because that’s going to give me a large market.
But what if my wife suddenly gets diagnosed with a very rare disease that only affects a few thousand people. At that point, “significant interest” completely changes. I’m going to focus my attention on my wife’s disease because of how invested I am in helping her.
In other words, determining a significant amount of interest is something entrepreneurs determine based on their personal goals. It can’t be quantified in a “one-size-fits-all” manner because every entrepreneur has different goals.