The RTCIceCandidatePairStats property firstRequestTimestamp specifies the time at which the first STUN request was sent on the described candidate pair.

A DOMHighResTimeStamp object indicating the timestamp at which the first STUN request was sent on the connection described by the described pair of candidates.

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

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