I would check in with them as needed. There is no specified time frame that is always “right”. You don’t want to irritate the buyer because you are always checking in nor do you want to fall off the face of the planet. Check in occasionally – perhaps once per month with a marketing update, a product line extension, some current industry facts/trends that keep your buyer informed, and/or to offer a special promotion. If you have nothing of significance to share, consider a slightly longer lead-time of 6-8 weeks until you do have something noteworthy to share. That all said, when you first land the account and your product is just hitting store shelves, it is a good idea to either email the buyer or send a card thanking the buyer for their support (and to remind them of your existence).