Friends’ kids broke my iPhone screen. I had tempered glass but the screen underneath also shattered terribly. How long should I be able to continue using this?

You could get it replaced for relatively little money at a reputable unofficial repair place

"Depends if you kept on hitting the kid after noticing the first cracks."

Should be fine, depending on the size of the crack and If it hasn't broken the panel and only the glass, otherwise depending on model and insurance cover if any... look at replacing the screen.

Just letting you know, but Apple will lie to your face if your screen doesn't work (when it comes to data recovery and stuff). They'll say that it's impossible to get the data back. I'm letting you know for when the screen actually breaks. I recommend going to a third party not affiliated with Apple.