The maximum Leaf value is 9,999,999. Chances are you won't come across this normally, so there is a trick you can spend to maximize your time. If you end up getting the ?Magic Frill? from the Archia Think Tank, you can use this item to quickly gather Leaf. A few things to note:
1) The first ingredient is the Refracting Cape. You can buy this from Ciela Gate's equipment shop Ladin't House (between the Dive Shop and the Inn) for 1,280 Leaf.
2) The second ingredient is the Silver Material, which you can buy in surplus at Archia's Filon Cross Bridge Town from the first screen's shop, Interfilon. Each will cost 202 leaf and you will easily earn back enough through this.
3) Each ?Magic Frill? requires 136 DP. I would suggest using a Reyvateil that you've got plenty of DP to spare.
The goal is to create x99 Lethal Frills (the default name for ?Magic Frill?). From this, you can then return to either of the two shopkeepers you use for this to sell it to. The total you'll gain from this is 686,070 leaf. Of course, you end up spending 146,718 for the total supplies (x99 Refracting Capes, x99 Silver Material), finishing up with a total of 539,352 leaf earned. Rinse and repeat.
*NOTE: It may help to wait for a New Game + when you've spent more time earning DP on your Reyvateils. It'll definitely help in the long run, as DP is easier to spend than it is to earn sometimes (unless you're in the end-game run).