TILA and RESPA under Reg Z and X respectively do not impose advertising retention requirements. Reg Z has a retention requirement but it excludes advertising. Other rules can apply such as FHA and ECOA (Reg B) restrictions on advertising, but those are not the question. TISA, under Reg DD has a two year retention requirement.
I recommend retaining all advertising from exam to exam at a minimum. It helps you demonstrate compliance with the advertising rules.