I assume this was already known, but Rift Herald doesn't have a timer for when he respawns via hitting tab in game. Guesswork be damned for how fast early game is now. Simple way to tell is kill herald, then hit tab to notice that the timer (Purple one) is on Baron's spawn, not herald's respawn.
Well, riot will probably add one, but in the meantime there's always the old method of typing "Rift Herald" into google and learning everything you need to know about him:
Location/timing:
Spawns at the Baron Pit
First appears at 4:00
Respawns every 5 minutes < - - - here
Despawns at 19:45 unless in combat; hard despawns at 19:55 no matter what
Encounter details:
Melee-only attacker - but hits pretty hard. Need to be able to take a punch to tank RH.
The Eye will occasionally open, exposing RH to bonus damage on attacks from behind equal to 12% of RH's Max HP as True Damage.
Takes 35% less damage from ranged Autoattacks.
**Reward details: ** 50 global gold to everyone on the killing team
Drops "Doom's Eve" buff to the killer: 10% increase to all non-true damage
Enhanced Baron Recall
Enhances nearby minions offensive power, same as Baron buff, with an additional attack speed increase. Does NOT increase their defensive power (don't worry, you won't have to deal with last-hitting Baron buffed minions in lane)
