The refill timer should be the same as vanilla, so 10 seconds per purchase. Is that not fast enough? If you want 0, you can simply use the Vending In Stock plugin to remove it globally.
If a feature were added to customize the restock delay, presumably users would want that to apply to all NPC Vending Machines, not just the ones that are customized via this plugin. That could be implemented, but it would expand the scope of the plugin to apply to all NPC Vending Machines, which I'm a bit hesitant to do unless it's absolutely necessary.
Before implementing a feature like this, I would like to understand precisely what problem you are trying to solve, since there may be alternative solutions that make more sense for your use case. For example, if you want to allow players to buy larger quantities of specific items, you could consider simply increasing the stack size of the sale orders for those items.
Come to think of it, I just remembered that I did implement an undocumented feature to customize the restock delay per sale order. You can add config options called "RefillAmount" and "RefillDelay" per sale order in the data file, like below.
{
"SellItem": {
"ShortName": "drone",
"Amount": 1,
"Skin": 2436737889
},
"CurrencyItem": {
"ShortName": "scrap",
"Amount": 25
},
"RefillAmount": 2,
"RefillDelay": 5
},
I didn't actually test this functionality so I'm not sure how well it works. RefillAmount defaults to 1 (order), and RefillDelay defaults to 10 (seconds). Keep in mind that since this feature is undocumented, it may be removed in any future update without notice, though I don't anticipate removing it since it was super simple to include in the first place and doesn't introduce any complexity to the plugin implementation (unless we add UI for it).