1. Standard - You have a certain chance of finding a rare every time you kill a rare eligible creature. This chance does not ever change, you always have the same chance of finding a rare.

2. Real Time Rares - When a ToD player kills their first creature they receive a timer. This timer can be any amount of time from 1 second to 2 months (average of that being 1 month). Once this timer expires (it expires at the exact time that was generated on that first kill if you've been logged in or not). Once this timer expired you get a very high chance of finding a rare along with your normal chance of finding a rare. This high chance of finding a rare lasts until you find your Real Time Rare (You can still find a standard rare when your real time rare timer has expired, this will not reset your RTR timer). When you find a RTR your RTR timer is randomly generated again to be another time between 1 second and 2 months from that point (this is from the time you kill the creature, you don't have to actually pick up the rare).

information direct from an Offical Turbine response.