I really hate how my phone locks after I use it for 4 hours-

I’m trying to learn a new song but it requires both my iPad and my phone to be on.
And I can’t learn a song under 4 hours!
ALL I NEED IS THE SHEET MUSIC TO BE ON, THAT’S ALL I USED MY PHONE FOR.
NOW I CAN’T FINISH CUZ IT’S GONNA LOCK IF I USE IT FOR 15 MORE MINUTES.
NO FAIR!
YOU’RE PREVENTING ME FROM DOING WHAT I LOVE!
HOW DARE YOU-