If you ever gain access to more 1st-level spell slots, such as with a multiclass, then hex is worth keeping your entire career, as the extra damage scales with your eldritch blast.
I have to agree - Warlock works really well with some low-level slots available from another class.
As a pure Warlock, you would need to get quite a few hits with hex for its damage to be greater than fireball (4 rounds worth of 5th-level eldritch blasts, for each target you would have hit with the fireball), I'd probably start using it less if not replacing it around fifth level. A hexblade who uses weapons and doesn't already have a magic weapon should swap out hex for Elemental weapon as soon as they get it.
Pure amount of damage, Hex wins. But that's not a good indicator.
Some math that Hex is more damage:
Hex does 0 on miss but is a rarely resisted damage type. Fireball is half on a save but is the most commonly resisted damage type. I'm going to call that a wash and just compare potential d6s. In other words, I'm assuming that the amount of damage lost even out between the two so I don't have to care about hit chance, save chance, resist fire, etc. Just a useful fiction.
Fireball is a 3rd level spell. Hex upcast to 3rd level lasts 8 hours. We'll consider that all adventuring day.
Fireball at 3rd does 8d6 in a 20' sphere. Considering placement not to hit allies, let's go with 3-4 foes hit. Sometimes will be more, sometimes less. More will happen with greater frequency, so let round that up to 4 foes. That's a potential of 32d6.
At 5th level, Eldritch Blast fires two beams. So Hex adds 2 potential d6 per round. (Pact Blade with the invocation also attacks at this rate.)
If the average encounter is 4 rounds, that's 8d6 potential d6s per combat.
With 6 encounters in a day, that's 48d6 potential.
So Hex, over time, at 5th give +50% the damage that fireball does.
(And 6 is the low end of 6-8 daily encounters from the DMG. Plus this ignores any Hex damage from reactions or bonus actions.)
Why Hex is still the worse choice:
Concentration - both takes up the slot and has a chance to be lost.
Need to use bonus action to move it as well as the action to apply it.
Can cast many fireballs in that time. Better nova.
It's boring to just spam at-will attacks.
Why Hex doesn't suck:
It does also grants disadvantage on one ability for checks, which can be useful if you have a grappler or shove fighter, a foe trying to do a STR check to get out of a web, etc.
It lasts 8 hours, so you can cast it and still have it up later combats after your short rest so its a free slot. Hex AND Fireball!
