As soon as you have a solid plan so you may whet the buyer’s appetite. The buyer wants to be kept in the loop. They don’t want to see your product on someone else’s shelf and not to be informed. Keep them “in the know.” That said, you might drop dime about your new product in a meeting or via email, but they may not be ready to really review it until their category review period.
Buyer’s are usually most eager to hear about new products during their review period. It’s when they are looking at their category as a whole, and are looking at the forest through the trees. That’s not to say they are not opening to hearing about it at other times, because you just never know when they will have an expected space on their shelf due to some other circumstances (think Tylenol recall and empty shelf space. No buyer wants empty shelf space).
Buyers (even those at the same retailer) have different review periods. National and many regional retailers review products annually or semi-annually. If you don’t know the buyer’s review period, ask him/her or their assistant.
When you do present your new product, the buyer will likely have a strong opinion about it and may ask you to make some changes to it in the way of the product itself or the packaging. Be prepared to have a solid plan in place, but not too solid that you are inflexible in the execution.