Typically you can get the feed when you get the worms. I haven't tried getting the leaves myself, I've only used the food.
I ordered all my supplies from Coastal Silkworm (it does make it nice that they are only 1.5 hours away from me. They have had great worms and I got the mixed food from them. I've gotten the pre-mixed stuff and the stuff I make myself (boy does that stuff stink up the house when you make it!)
What I have found is that depending on the amount of food you give them and the temperature they grow at different rates. Also a big factor is how big they are when you get them. Small ones will of course last longer maybe around 1 month or longer before they cocoon. Big ones could be just a matter of a week or so. There is lots of info on the web about the growth process and the different stages they go through.
The last bunch I got ate through their food really fast and I didn't have any more and they stayed the same size and lived for about a week until they were all fed off.
From what I have read on this site they should only be fed as a treat. So maybe a couple a day or every couple days. To be honest I am not sure why, maybe they are too fatty or a poor calcium ratio passably.