A cycle is a unit of time measurement used by Romulans. One cycle is approximately equivalent to no more than 29.35 Earth minutes.
A Romulan officer noted they had no motion on their sensors for 20 cycles. Kirk had also noted in a log that the Enterprise had been motionless for 9 hours 47 minutes. (TOS: "Balance of Terror")
- In the background chatter of TNG: "Unification, Part II", it is possible to hear dialogue noting something about "two cycles". The exact line is "We are very close now... within two more cycles."