The formula for upkeep is the same for everyone. You start getting upkeep after you reach either level 25 (or some lower level, used to be 25, but might be lower now), or you have more than 10,000 gold in your account.
The formula should be: for every tick you get (experience and gold) you have a 4% chance of repairing every item you spawned with. If you get hit with the repair, it will be 7% of the total cost of the item. I believe that is the worse case scenario. I've noticed, and heard devs or admins say that you may have less of a chance of getting repairs if you have a higher multiplier, but I don't know exactly how it works. I've noticed when I have higher multipliers that it seems like I get repairs less, but that could just be speculation.
One thing I do know, but not the exact %, is that the higher your WPF in a weapon skill, you have a lower chance of repairing those types of weapons (so if you have higher 2hand WPF and carry a 2h weapon, you have less than 4% chance of repairing that item).
And you have to remember it's 4% chance, out of infinite. I'm not a math nerd, but that means you could technically be hit with repairs 100 times in a row, and although that's a 100% if you take it out of 100, out of infinite it could still be 4%.