Seresto collars are water-resitant, so they remain effective following a bath (with or without shampoo treatment), or even after your dog goes for a swim. Therefore, there is no need to remove the collar for bathing. However, using a gentle, soap-free pet shampoo is recommend.
Under normal conditions, the collar's effectiveness should last for 8 months. In order to make sure it lasts this long, you shouldn't bathe your dog more than once per month.