Count conversions, not days
A campaign with three conversions has told you almost nothing regardless of how long it ran. The same campaign with forty has told you a great deal in a week.
The working rule
Give it enough spend to produce a meaningful number of conversions at your expected CPA — or enough spend at two to three times your break-even CPA with none, which is also an answer.
What not to do
Judge during the learning period, then change everything, then judge the result of that. Every account we inherit in trouble has that pattern in its change history.