The length of time it takes to boil sweet potatoes is around 20 minutes.
The sweet potatoes will be boiled and ready to serve or eat after 20 minutes of cooking time.
When the sweet potatoes are soft and easily pierced with a fork then the sweet potatoes are ready to eat.
As long as you boil the sweet potatoes for at least 20 minutes they should be done enough.
If you need to boil the sweet potatoes longer then you can do so.
But for me 20 minutes of boiling the sweet potatoes is enough for me and they become soft enough.
The same goes for regular potatoes when I'm making boiled or mashed potatoes.
Boiling the potatoes for 20 minutes makes them soft enough to mash.