Whats the lifespan of a fat cat?

HI I got my cat timmy about a year ago off kijiji (website similar to e bay), the previous owners said he's about 6 or 7 years, he's fixed extremely obese and lazy, thats the part where i'm concerned he's about 25 to 30 lbs so big his nick names tubz, he breathes heavily but his teeth and gums look fine 2 me but I obviously wanna visit a vet asap, he had worms a couple months ago so we just bought some drops from the pet store & it looks 2 me like there gone but once again I wanna check a vet.... So my question is whats the lifespan of a fat cat like my tabby tubz <3 also if anyone know some tips 2 help the cat loose weight or indications 2 possible dieseases........ This wasn't orignally my cat I inherited it from my mom so i want him to have the longest life possible for a lazy obese house kitty that ocassionaly frolics in the grass. I know theres cat people out there! so PLEASEwould appreciate an outside opinion