> get-cluster | fl *subnet*
CrossSubnetDelay          : 1000
CrossSubnetThreshold      : 20
PlumbAllCrossSubnetRoutes : 0
SameSubnetDelay           : 1000
SameSubnetThreshold       : 20

Heartbeat timeout is SameSubnetThreshold.

SameSubnetDelay is in millisecond. So along with SameSubnetThreshold what this means is it will ping every 1 second 20 times till it gives an error. So for 20 seconds if there is no response, then it will give heartbeat timeout.

Update

(get-cluster).SameSubnetThreshold = 30

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references:

Tuning Failover Cluster Network Thresholds - Microsoft Community Hub