Suzuki gives 2 years warranty for Wagon R. You just visit any dealership, they will check their records by engine number. Warranty will be exactly two years starting from day of delivery. However you can extend warranty for two more years i.e. Total 4 years but they may reject extension if you've not perform regular services from authorized Suzuki. They rejected my application of extended warranty for the same reason.
My wagon r 2017 model is giving sometime 15 kpl and on motorway/highway 17kpl. Average mostly depends upon driving habit. It gives different average under same conditions for different drivers. Oil also affect average. I had been using 10w-40 but I'm thinking to switch to 5w-30 as suggested by many on this forum.
Visit the authorized dealership, they will check their records by engine number. They may be able to help you even if you don't have the documents. Also, any previous receipt, delivery challan may also help.