What's the opposite/inverse of "rate"?

Might be better for a math or datacomm exchange, but I'll try here first:

Given that "rate" means the number of times that an event occurs in a unit of time, what is the opposite/inverse of that term? Ie, what is a term for the time between events?

"Latency" comes to mind but is not quite right since it generally refers to the time between cause and effect, not between essentially identical events.

Solution 1:

If something is happening repeatedly, we say the value for rate (or frequency) increases if it starts happening more often in a given time-span. Conversely, if it starts happening less often we say the interval is increasing.

interval - a space between things, points, limits, etc.

Solution 2:

In economics, the term in [sparse] use seems to be inverse rate. Otherwise, it looks like the English language doesn't have an appropriate word.

In electrical engineering, a similar concept is conveyed by resistance (the inverse of conductance), which is a measure of how much effort is required to achieve a flow of current. EE's have developed a set of mental tools to manipulate and work with these concepts.

The word interval works for quantifying discrete events (e.g., minutes per customer), but it just feels weird when quantifying continuous values (e.g., hours per kilometer).

It seems prudent to define a word in the language to convey the concept. It's just silly that so many questions on the GRE and GMAT revolve around manipulating inverse rates, as if it's hard to do. It's only hard to think about when we don't have a simple word to describe it.