The block explorer reorg log
is showing 15 reorgs in the last 8538 blocks. We generally assume that about half the forks lead to a reorganization in a given node*, so that is about 30 forks. That is about one fork per 284 blocks, which is close to my estimate of 300 blocks per fork.
So, I would expect a two block fork every 90 thousand blocks or so, maybe every 80 thousand using the block explorer data. That is every year and a half, by the way. A three block fork should show up under honest circumstances about once every 450 to 500 years.
A shorter block time target would probably lead to more frequent forks, measured in blocks per fork, but it isn't obvious what the function would be. Halving the block time target, for example, would lead to probably more than double the forks per year. It could probably be simulated, but hasn't that I know of.
* The best predictor of which block will win in a fork is the fraction of the network seeing that block. If we assume that the distribution is more or less random, they should both average out to around 50%.