There is no rule on how often you should replace the contents of a first aid kit. But, as a responsible first aider, you should make sure to check yours regularly. Start by looking out for expiry dates or anything running low for replenishment. Also look out for signs of damage including rust, loss of adhesive properties, fraying or tears.
Depending on whether you intend to use your first aid kit for domestic, workplace, or sports uses - the items may differ. Good staples include plasters, antiseptic wipes, bandages, dressings, and gauze. Also, make sure to keep your first aid kit in a safe, dry place that's out of the reach of children.