How long is overtime

"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.

As a rule of thumb, no.

The situation may differ depending on whether you're attacking or defending, but the time it takes to die, respawn and run back is time you could be spending diverting enemy attention, rounding up health packs (to zone/deny your enemy getting them when the next team fight starts), or scouting for teleporters/turrets/flankers.

One thing I will mention though, is that in a situation where it's suicide or die to the enemy without the option of retreating/regrouping (let's say you overextended and can't backtrack without getting pipped by a turret or sniper, and you're watching a Reinhardt trudging towards you); suiciding is the better of the two options.

This is because if you let that Reinhardt hammer your face in, he gets ultimate charge, which stacks the odds against your next engagement. If you suicide, the respawn timer is the same, and he is ~10% further from his ultimate because you decided the cliff would treat you better than his beatstick.