so that after the player has held space for 0.25 seconds, jumping = true
just use update() function .
and in update use something like
public bool jumping = false;
void update()
{
if(Input.GetKeyDown(KeyCode.Space))
{
Invoke("TheFunction",0.25f);
}
}
void TheFunction()
{
jumping = true;
}
Not tested but , looks like it works
let me know if it works or not .