It follows the 32-bit limit rule, a value cannot go over 2,147,483,647, thus you cannot more than 2,147,483,647 worth of credits on a single character.
Take into account the cost of your ship, equipment, and credits on that character. I play it safe and keep only 999 million on any one character. I dont have huge cap ships with top armor so i am safe... i hope o_0