PriceYak typically will catch a price change within 20 minutes.
PriceYak only creates a revision on the listing if it finds a price or availability change on the item. If there's nothing new to report, PriceYak will not create a revision.
What if a price changes between runs and someone buys?!
We've processed millions of orders and it's extremely rare that the following happens together:
a) The Amazon price changes before PriceYak catches the price change
b) Someone buys from you in that same window of time
In these rare cases our customers have handled this situation in the following ways:
1. Cancel the order, explain that the item isn't in stock, and apologize to the customer.
2. Source the item from another eBay seller, who doesn't use PriceYak ;), and ship it to your customer.
3. Buy the item at a loss to make the customer happy.
Before using PriceYak, (i.e. running arbitrage by hand) this issue used to occur very frequently, but we've greatly improved upon the manual process and we think you'll be happy with the results.