Hey it is called the thumb rule, Change after 3 years irrespective of kilometers used.
Stock tyres on the Ritz are JK Vectras. They last about 30-40k kilometres depending on how and where they've been used.
As I understand you wish to replace the tyre of your Ritz. we would suggest you to stick to the one which has been recommended by the manufacturer i.e Maruti Suzuki. You can also go ahead and contact your nearest service center for this information. Click here to fetch the information of your nearest service center.
Maruti has a good and vast service network and there will be no problem of maintenance and replacement of parts at later stage but the resale value of the car might decrease as the company has decided to discontinue the vehicle. Despite this, there will never be any shortage of parts as manufacturers make parts for next 10 years. Also, as Ritz share a good number of parts with its sibling Swift there will be no shortage whatsoever. If you would like to consider, the Indian auto market has many other offerings in the same segment such as Tata Bolt, Nissan Micra, Ford Figo, Honda Brio and Grand i10, Maruti Swift, Chevrolet Sail hatchback.