Depends on what you want to measure or how you created the workout.
I did a Garmin coach training plan recently and the hill repeat had lap press at bottom of hill, run up for 15 seconds, walk for 60 seconds, run down for 15 seconds. Repeat. So, only one button press per repeat.
It says that should get in position at the bottom of the hill and press the lap button. Then you follow the instructions. Run 400 meter uphill, run 400 meter downhill and then recover for 1 minute. After the recovery you got how long time you like to get to the bottom of the hill for the next run. Only press the lap button when you are ready to start the next hill repeat and are standing at the bottom of the hill.
If you press the lap button during any of the steps the watch will go to the next step. So you can cut the recovery short if you like or if something happens during the sprint you can do a shorter run.
