Arrendell

For me, if the accumulated time in the sun throughout the middle of the day was going to equal more than 30-45 minutes then I'd be applying sunscreen once in the morning and then again early-mid afternoon. It would take literally 30 seconds and I'd feel protected. 

once a tattoo is healed the sun will not hurt it,
that being said, keeping yourself protected from the sun is a life long thing, skin cancer is a real concern, so sunblock if you will be in a hot sun for a while, etc etc, UV protection is also ongoing, just good practice to shield your skin form the sun, but it will not damage a healed tattoo.