On your character BP create a float variable called start time.
On the event begin play, right click the event graph and find get game time in seconds and set your float start time to this.
I guess next you would need a overlap event to trigger once you compete the lap and in this event you need to get game time in seconds again and store that to a new float called end time.
Simple macro to do float - float. End time minus start time = time to complete race.