If you use the welcome letter we provide, there is only one welcome letter available. It is editable, and you can view it by logging into your account by going to the Contacts tab, and then selecting either "Grow My List" or ""Sign-up Tools".
Some users choose to supress the welcome letter in their APi integration by making the action by parameter 'action by owner', which is the default action taken by the API if no parameter is set. You can then generate your own welcome letters (campaigns) and write code to control what letter gets sent out to each contact.
You could do this by creating two campaigns every 85 days, and triggering a resend through the API whenever someone is added to the list that the campaign is intended for.