I think pretty much never. But anyway, I do think 10 games is reasonable. As form changes over time it's difficult to say which number to use actually. Arguably fewer is a better indication of current form.
Anyway, the point is 2 of those games we lost when our reserves played. I'm not sure that says anything about the form of the first choice players and both times we won the following game when those players returned. Another loss was City away. As you well know City have not dropped a single point at home in 9 league games. Losing there is not an indication of poor form and, again, we won the next game.
Stats need some analysis and interpretation, they don't just stand on their own.