You can do this immediately at the beginning of the game. I would dip into the first world and get some herbs/spices you want to copy, though. Keep in mind that Thomas cannot have any of the item on him you are duplicating, so check his inventory first so you don't waste time
...Equip the Nexial Binding in your quick selection "belt" or whatever you want to call it. Have the Binding displayed in your "belt". Walk up to Boldwin, and prepare to buy something (Crescent Moon grass for example).
When the purchase screen either tells you you can't afford it or asks you how many you want to buy, quickly move next to Thomas. DO NOT CLOSE THE PURCHASE WINDOW. The window will disappear on its own, and Boldwin will either yell at you, or say nothing at all.
When he's done talking or not saying anything (whichever happens) mash square to activate the nexial binding. Don't actually use the item, but instead press the start button on the dualshock to clear the 'do you want to lose your souls etc' screen. Now quickly talk to Thomas and deposit the item you want duped*. As soon as you confirm deposit, you'll activate the Binding, and you might notice your item's count increase by one in your inventory before the loading screen.
Return to Thomas, and you'll see 99-1024 of whatever you dropped. Rinse and repeat. It might take a few tries to get it right.
*You can do this with the following (off the top of my head):
Demon's Souls (including Colorless)
All consumables (including arrows, in case you were wondering why I had 300 white arrows
)
Upgrade stones
