Different self sealing tubes and different sealants use different chemistries so it depends. Stans No Flats and Zefal use a latex liquid that has a life of about six months. Slime recommends their 'product' be replaced every 2 years. Flat Attack guarantees their 'product' for FIVE years.

Am currently running Schwalbe Marathon Supremes (anti-flat tires) with Michlin Protek Max tubes (self sealing) and Maxxis Hookworms and Geax Tattoos with Flat Attack in the inner tubes. Self sealing tubes do tend to hold their air longer between top-ups. Aside from that - nothing to report - as in no flats in three years. Can't say if I've been lucky or if the 'insurance' paid off, but I currently don't carry a repair kit, and just got back in from a 120km outing.