I've just started the game. I'd say a couple hours to get to Jeet's (you need to follow the storyline until you go to explore the Jaws) and collecting all 4 pieces to build the Scrap Team or whatever is called in english. Then it'l give you like 50-60 scrap per hour offline, more or less.
If you progress on the story you can build more Scrap Teams, speeding up the process.
What you can do is checking always that your stats tab matchs the challenge tab. Check the video on the first page. Then, when you're done with this challenge, just make an usb backup or cloud backup just in case.
I'd say you should worry only if you die upon completing a scavenge zone, but better check everytime.