To reshape thermoplastics they need to be at what's called the glass transition temperature  ( Tg )

the Tg for the polystyrene used in these kits is 212F ( 100 C )  ,, coincidently the same temp as boiling water .

Those blades are so thin they won't need to much time in the boiling water  .

15 , 20 seconds ?

You can't overheat them since the boiling water will never go above the 212  , but I'm sure you know that .

Wear Gloves ,