jaspud on "[Plugin: s2Member Framework (Member Roles, Capabilities, Membership, PayPal Members)] s2Member: Paypal payments recurring 28 days"

You would have to test this but I think if you change these two numbers in the generated shortcode it would work for 28 days.

rp="1" rt="D" (This is for 1 Day as generated by s2member)

rp="28" rt="D" (I would guess this would work for 28 days).