It is an action to break a grab, it would be the same to break out of restraints.
Breaking or escaping manacles is a relatively quick process for an adventurer. A Fighter is just going to flex his muscles and strain a bit and the chain will break. The rogue is going to flex her fingers and wiggle free (or pick the lock, disadvantage if they are trying to pick their own handcuffs).
There's no reason to charge more than an action for breaking of manacles. Consider for a moment that they have 15 Hp, that's about what 1 attack does on a crit and manacles are likely incapacitated.
It should be noted, that if you think this is not a difficult task, that a 20 strength check is not easy, there is no skill qualifier, so there is generally no proficiency bonus. So at most your average character with a 20 strength score, is going to have to get a 15 and a normal character has to crit (exceptions are the Bard, and Champion fighter who get half prof to non prof checks in all, and specific abilities respectively). That seems about right to me.
As far as how many rounds it will take on average to break a set of manacles...We'll look at Str +0, +3, +5 and +8. Those seem to be likely common str check modifiers.
- +0 1/20 shot, 20 rounds on average. So about 2 minutes.
- +3 4/20 shot, 5 rounds on average. About 30 seconds.
- +5 6/20 shot, 3-4 rounds on average. About 24 seconds.
- +8 9/20 shot, 1-2 rounds on average. About 12 seconds.
That looks pretty good to me. It's not easy, but it's doable for a heroic character.