If you did the most damage on a creature it will take 10 seconds for someone else to loot it as everyone will be able to loot it even if they didn't make any experience. For the loot from bosses where it's a reward bag, this can be different based on the reward system. For example, the mutated pumpkin from my understanding you can loot right away however if it's a boss like oberon then you will have to wait 10 seconds Bosses like Oberon also fall under this category..(Thanks Fire Dragon in the comment below)
For some bosses in the reward system with a reward bag your loot would be secured in your reward chest and no one will be able to steal your loot if it's in this type of reward system like the Mutated Pumpkin.
Some bosses if you didn't make any damage or qualify you won't be able to open your bag if you didn't make the most damagea reward bag even.