Basically what @314 said, If I was processing contracts (Pretend I'm still staff) and I saw a 100K loan have late fees of 500K per day for a year, I'd ask them to change it. Late fees have to reasonable and fair as far as I know.

 

When you create loan contracts consider "Can this person earn this much with just 2 hours of money making a day?"
And late fees should be the same. We're going to assume the average person can make 20-25k in 2 hours. upwards of 30-35k if they have a melonsword/eff7. But let's assume the average person with max tools.
You give this person a 100k loan. This, if broken down into days of 2 hours is 4-5 days. This can be considered reasonable, but if you want to be sure to be paid back, assume people make less money during weekdays than during weekends. Therefore you should add 2 more days. 100k / week is a reasonable amount.
Latefees are basically the same, but you should only consider what they could make in 1 hour. so 10-12k an hour for average people, or 15-18k an hour for people with melonswords/eff7.
And never go over 1 week with late fees, because usually you'd just be an ass if you did. (and risk getting the contract denied)

To conclude:
With loans you should assume 100k a week can be paid back reasonably by almost everyone. People with eff7s and/or melonswords might be able to do 120-140k a week. But to be certain you get your money back you should do reasonable loans of 100k a week. (paying back 100k a week, that is)
And late fees should never be more than 10k a day, when someone's online an incredibly large amount of time you can try 15k a day.

Of course, individual circumstances are different and I always urge people to discuss how many hours a day the person you're lending money to can work to make money. If they can't work for more than 1 hour a day, then you shouldn't demand they make an average of 20k a day. And never believe anyone who says they can work for longer than 2 hours a day. Most people can't. Believing that is asking to be scammed.