How to best visualize or capture time interval between lab measurements?
The average or median seem reasonable here. A boxplot might be a good choice of visualistion:

The y-axis could be your time dimension, and boxplot shows the normal distributions of differences, including the median value and outliers. You could then compare certain measurements against this. But it would of course depend on if you have enough data in your sample to represent the total population.
Or simply calculate the standard deviation of differences, and report this with your median (the mean value may be warped by outliers).