Currently (Requiem+Pandemonium 1.5.0 and Minecraft 1.16.4), when you sting someone as a Bee, you are given a limited amount of time to live (like a regular Bee), but that is not shown in the HUD. There are a couple of ways this could be fixed:

• Show the vanilla hunger bar. The value (number of food points) starts at 20 and counts down by 1 every 3 seconds after you last sting. Once this value reaches 0, the bee you're possessing dies.
• Show a new timer in the HUD that starts at 60 seconds and counts down. Once the timer hits 0, the bee you're possessing dies.

Thanks for the suggestion ! It could be a neat addition, but it might be a while before we implement HUD improvements for specific mobs.