Yup you put the rat in and wait. You will know if the snake is interested. He will coil up, flicking the tongue a lot and probably start searching. Oh trust me, they can smell a rat a ways away. All my guys know there is dinner coming when I walk into the room with their dinner in boxes. I feed live but I pre kill all mine before feeding. So the rats come home alive and boy, they can all smell them. Rosey usually is never in her hide and she is at the door, Dottie is standing up out of her hide like a little cobra ready for dinner and Vicky has her head poking out of her hide waiting patiently but she is a very elegant eater. She squeezes the mouse, then inspects it for about 5 mins, then slowly eats it lol. Well as long as your snake feels comfortable and the temps are right, he should eat. I've never had a snake willingly go 3 weeks without food but its not uncommon for them to. Good luck.
Another thing you can try if that doesn't work is kill the rat(unless you can actually return it) and then just put it in the cage at lights out and leave it in there overnight. Some snakes are shy and wont eat until its all dark and quiet.
Oh also forgot to kind of answer your question, if it doesn't show interest in about 15-20 mins, I would remove it. And removing the hides might or might not make the snake scared depending how comfortable your snake feels to begin with. Like I said, I would try with the hides in. You will know if your snake is interested.