I always believed that the chance to find an item is calculated per 1 h and it is in the vicinity of 24% per hour at 100% motivation. Therefore if you do 10 min intervals, you basically have 24/6 = 4% chance to find an item. However I am not sure at all about that because with this math a 15 sec job would only have 0.1% chance to find an item and I can tell you that on average I find 3 items per 75 15 sec jobs at motivation higher than 75%. Which would indicate that 15 sec jobs have an item drop rate of around 4%
It used to be linear before 2.0, but now I doubt that it is since none of the other rewards are. As for the exact percentage, the best people can do is guess.