60 seconds is short
It’s up for every all in fight. It will be up in every teamfight. It has a shorter cooldown than most ults. And, if you take ingenious hunter (I think it was named as such?) it can drop to a 36 second cooldown. It’s a VERY strong ability. And if anything, the ability for it to gain stacks from spellcasts makes up for the time when you aren’t using it when it’s off cooldown. In the case of someone like Casseiopeia or Karthus, you should easily reach close to 100 stacks by the time it’s off cooldown, and in the case of Champions like Kassadin or other assassins, it accomplishes its goal by giving you mobility and AP before you burst someone.
Do note that this item gives the most possible AP from a slot for 4 second. Giving 200-300 AP (or 280-420 with deathcap), making it the most slot efficient item in the game for those 4 seconds. And even when the active isn’t up t still gives 100 AP and 10% movespeed.