Also not to be a downer, but there are tons of time tracking apps in the market.
It's saturated, and honestly not a big pain point to solve for most people.
Customer profile that make sense and might retain: freelancer, agency that employ freelancer.
General public will find time tracking cool, but they majority won't use it day to day because it doesn't solve a problem.
Or it solve a problem, but resolved in a short period of time.
You can use app.sensortower.com (pretty accurate with some margin of error) to benchmark revenue for similar apps—at least those that released on App Store.