This depends heavily on stage and product but a few rules of thumb, assuming you're starting out.
You need to prove that a specified number of monthly active users (whatever your primary KPI is) understand, want, and will continue to use your app. A few KPIs on each:
- Understand (unique value proposition or UVP): Met if >50% of individuals state the same UVP in a sentence as your internal team
- Want (net promoter score or product-market fit): Met if over 40% of active users would be extremely disappointed if they lost access to your product (as opposed to somewhat or not disappointed at all [see Sean Ellis' 40% rule])
- Continued use (retention): Met if we retain 85% of active users month over month
If you hit these for a predetermined number of paying MAUs (say 200 for ex) then optionality will come and institutional funding will come to you.