Time IMO has nothing to do with the switch. Its how your present the prey to the animal. First make sure it really warm. Once the rat is thawed I usually drain the water, and add new hot water to increase the temperature right before offering. Snakes go off of heat and smell, so its important that it is thawed properly. Once you are ready to lower it into the enclosure, grab the skin behind the neck, and make it act like a live mouse/rat. I usually wait for the snake to come out of its hide, and once I know its interested I slowly wiggle it and bring it closer. Sometimes the snake will want to check it out first, and give it a little sniff before jumping it. Just make sure its always moving, and don't just shove it in the snakes face. This is what a lot of people do when transitioning, and to me is why it results in such high failure. I personally was able to transition ALL of my snakes to f.t. All were live feeders, but due to a couple of pet stores closing down it became hard for me as well, so I switched.

Another thing I did for ONE of my snakes was give in and feed it live anyway. If you can't find then this obviously isn't an option, but if you can I think it works wonders. Once they are used to the enclosure, just feed them a live prey item. I would do this for about a month, and then after a few feedings when you know he's ready to eat offer a f.t. Most of the time they are so used to you feeding them live as soon as they see the thing its gone. Good luck, hope this helped.