You do not have to prepare to make up for any karma. Just think of this. Can you go back and change it so you made the opposite decision? No, you can't change it. And to lay to rest your question on whether to ever try to go back and fix things: ask yourself if you went in with the best of intentions, would you improve all their lives as a whole? If you're a good person, and they could forgive you, then maybe yes. If you will do more harm than good in you're opinion and you would be serving yourself to the detriment of others, then maybe not.
But as for how long it will take you to work off any negative karma from that situation? Zero. Just ask yourself if you would ever do that again, if no, then there is no reason to have any negative karma or guilt. It's a moment past in time, you can't change the past. You can reset your karma to 0 in this way at will. I use this when people try to guilt me when the feeling of guilt doesn't serve me, so I choose to ignore it.
Seriousness aside, welcome to the forums! The more people that join, the more insight and information is gained by all of us.
But as for how long it will take you to work off any negative karma from that situation? Zero. Just ask yourself if you would ever do that again, if no, then there is no reason to have any negative karma or guilt. It's a moment past in time, you can't change the past. You can reset your karma to 0 in this way at will. I use this when people try to guilt me when the feeling of guilt doesn't serve me, so I choose to ignore it.
Seriousness aside, welcome to the forums! The more people that join, the more insight and information is gained by all of us.