"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.
The following are all things that I've seen work for me and my teammates, playing from early in the closed beta to now.
Reinhardt's shield and Winston's bubble both block the hook, so if they can be raised before the hook hits anyone, that can save not only the player, but also their teammates.
As one of the other answers has said, using a character that has an "out," like a dash, you can semi-reliably escape from the Roadhog.
Dashing with Tracer or Genji, or using Tracer's Rewind, might save you if the Roadhog isn't fast enough. Likewise with Pharah's Jump Jet. McCree can try and stun the Roadhog and roll away, which might work in his favor.
Reaper and Mei can easily use their invulnerability abilities to help prevent their demise. Unfortunately, Mei's leaves her right there next to the Roadhog, so you have to hope your team can save you. Zarya's barriers both block the hook, if applied before it lands. Also, Zarya can activate a barrier on herself or an ally after they've been hooked to hopefully ensure the Roadhog does not get a kill, and instead gives her a big boost of energy. Call out to a Zarya on your team if you have one, and they might keep you alive.
Winston and Zenyatta can pop their Ultimate to pretty much ensure they survive the encounter, but that's probably a wasted ult.
Lastly, one thing that works for me about half the time, since I play Lucio very frequently, is to use a knockback ability (Lucio's alt-fire) on the Roadhog, which sometimes prevents a killing blow from landing and occasionally disorients the Roadhog. This can also work for Junkrat and his concussive mine. Press shift and right-click immediately. The Junkrat will be blown back, and the Roadhog might even die. If Pharah's Jump Jet is on cooldown, her Concussive Blast would knock both characters away from each other, and might be more effective than Lucio's alt-fire.
One important note with using knockback abilities: Junkrat's and Pharah's abilities move both players, so they usually work a little better in certain situations. If the Roadhog hooks you as Lucio while standing backed up against a wall, trying to blast him away with your alt-fire will be much less effective, if not pointless.
Something that could work against a very slow or just bad Roadhog is Hanzo's scatter shot, fired at the Roadhog's/Hanzo's feet. I've never seen this successfully done, but theoretically it might work.
One final note: Widowmaker's grapple probably wouldn't work, as it is very slow, but poisoning the Roadhog with a venom mine as you die might ensure that he goes down, too.
Best Answer
From my experience, scrap seems to last forever until I pick up a certain number of them. I am not certain what this number is since scrap still exists around the map even when I have reached the full 200 scrap limit. The game does indicate where the scrap is located with two downward arrows being shown throughout the map (can even see them through walls), but I have noticed these indicators disappearing after a certain number of picked up scrap (or possibly time, but I am going with number of scrap items - each giving you 25 scrap). As for armor packs, like Powerlord said they last forever as long as your teammates are not hit. Obviously this armor deteriorates first with enemy damage.
The Overwatch wiki pages don't really mention how long scrap lasts on the map without being picked up. Source: http://overwatch.gamepedia.com/Scrap