I think a Poisson distribution is what you're looking for here. Roughly, if events happen independently and with a fixed probability per time interval, you get a Poisson distribution. Poisson distributions apply to a lot of things, so it's a very useful distribution to know about.
But since the number of cars is increasing, it's not a Poisson distribution; if the chance per car per time is constant, you'd expect the time to the next fire to be shorter.
Presumably it will go up to some degree over time as the average age of tesla's fleet converges with the industry average, as older cars are more likely to experience malfunctions/breakdowns for obvious reasons.
The median car in the US is ~11.6 years old[1], while Tesla's oldest vehicles were released in 2008, and the vast majority of their fleet was sold in the last couple years.
Obviously, the massive differences between electric powered cars and internal combustions engines means that they may never reach parity, but unless Musk has figured out a way to beat entropy, its pretty safe to assume that older cars will break down/suffer leaks/explode more than newer ones.
