Overwatch – Does Speed of Capturing Objective Depend on Number of People?

"Objective time" is how much time you spent actively contesting the objective.

On attack, this means how much time you spent escorting the payload, or standing on a capture point (that your team does not actively control).

On defense, it means how much time you spent actively stopping the offensive team from making progress with the objective by contesting it with your body -- so, time spent touching either the payload or capture point while the enemy team also has someone attempting to push or capture it.

Since the roles of attacker and defender often switch during Control maps, Objective Time on Control is a combination of both of the above metrics, depending on whether or not you own the point at the time -- so you gain time only while actively capturing or contesting the point, and not while standing on the point with no enemies attempting to take it.

From Overwatch latest PTR patch notes :

Competitive Play

• Fixed a bug that could cause overtime to continue indefinitely on Assault maps

So yes, if you are unable to recapture a point, after a certain amount of time, the game will instantaneously end, giving the win to the team that currently controls the point.