This is the amount of time, after a mob is killed, that it is expected to respawn.
Some named mobs, such as world bosses, may take as long as 12 hours (12:00:00) or even an entire day (24:00:00).
The actual respawn time can vary by a randomly generated value of 30%, plus or minus, of the stated time. So, the GDRT may be 1.5 minutes but the actual respawn time can be anywhere from 1.05 to 1.95 minutes, or 1 to 2 minutes. A mob with a 1 hour respawn timer might appear anywhere from 40 minutes to an hour and 20 minutes. This is to help reduce the incidence of "spawn camping", especially with Elite and Boss mobs.
All Respawn Times are stated as Hours:Minutes:Seconds.
Global Default Respawn Timer
The GDRT in RoM is 01:30. Any mob with a blank respawn= value will display "00:01:30 GDRT". This is rather faster than most MMORPGs so it is advisable not to go AFK while out in hunting areas. Many Elite mobs respawn in 03:00. Crown Elite World Bosses tend to respawn in 12 to 24 hours.
If you do NOT see GDRT to the right of the time this means that respawn= has a value and is not simply displaying the default.
World Boss Respawn Variety
The Respawn Time of all World Bosses will automatically vary, plus or minus, by as much as 33%. This is to discourage Spawn Camping.