I did some math and found that the 90% confidence interval for bitcoin blocks is that the block will take less than 1381.6 seconds. I used the calculation ln(10)*600 with 10 being used because a 90% confidence interval means one in ten blocks are expected to fall outside of this range. Could this be considered a 90% confidence interval?
Note: this could change if the difficulty is expected to rise or drop. If block time is 576 seconds, for example, it becomes a 91% confidence interval. In practice this changes a lot once the difficulty adjustment has passed.