It seems like all the high-profile games that come out for Switch are going to have battery questions attached to them. People wanted to know how long you could play Breath of the Wild on the go, and now those same questions are coming in for Mario Kart 8 Deluxe.
We don't have a ton of details for you, but we do have data from one playtest. The test was performed with a full charge on the Switch, and the screen was set to 30% brightness. According to the tester, the Switch lasted for 3 hours, 9 minutes, and 20 seconds before it gave up the ghost. In other words, it seems like MK8D should be right in line with what you got out of Breath of the Wild.