The RTCIceCandidatePairStats property lastRequestTimestamp indicates the time at which the most recent STUN request was sent on the described candidate pair.

A DOMHighResTimeStamp object indicating the timestamp at which the last (most recent) STUN request was sent on the connection indicated by the described pair of candidates.

You can use this value in combination with firstRequestTimestamp and requestsSent to compute the average interval between consecutive connectivity checks:

avgCheckInterval = (candidatePairStats.lastRequestTimestamp -
                    candidatePairStats.firstRequestTimestamp) /
                   candidatePairStats.requestsSent;