Rate of Shadow gain depends a lot on what foes they're facing. Whatever the cause, what your player-heroes will want is HARDEN WILL on page 137:
In game terms, Player-heroes whose accumulated Shadow does not yet match their maximum Hope score may choose to remove all their current Shadow, replacing it with a single Shadow Scar.
This can be done as needed, and there is no firm ruling (yet) on how long it takes to "tap into their innermost reserves of spiritual energy to harden their determination, before it’s too late." As an LM, I would probably err on the side of generosity and make it a secondary action, but I could see an argument for making it a primary action or something only done out of combat.