Is this Homebrew Eldritch Invocation, Accursed Memory, balanced?

Accursed Memory, Prerequisite: 5th level

You can cast Hex at will as a 1st-level spell, without expending a spell slot or material components.

This invocation is pretty straightforward, but, I suspect that the nature of Hex may present balance issues compared to other invocations like Fiendish Vigor or Armor of Shadows. At 5th level, a Warlock may use a single spell slot on Hex for 8-hours assuming they maintain concentration. This invocation would save them that 1 spell slot. However, the invocation would also allow Warlocks to break concentration without worrying about “wasting” that precious spell slot. Over the course of the day, this could be very useful for a Warlock…maybe too useful?

Alternatively, limiting this invocation to “once per short rest” or “once per long rest” is an option. Although, I’m not sure how useful 1 free casting of the 1-hour version of Hex would be when a 5th level Warlock can already cast Hex at 3rd level for 8-hours.

Any help would be much appreciated!