How long does 50ml of sunscreen last you?

So I've been really inconsistent with sunscreen because I felt like it wasn't really working. I still felt like I was burning in the sun. Untill I recently found out that not only are you supposed to apply 2 fingers/a quarter teaspoon of sunscreen, you actually have to reapply it!

But after doing some maths it seems like one tube of sunscreen really wouldn't last very long. Many sunscreens come in 50ml. So if you're applying 1.25ml every 2 hours and you're outside from 8am - 6pm, you would use 10ml that day. The sunscreen would only last 5 days! That seems a bit unrealistic, so do you guys really reapply this often?

If you don't, please tell me how you're applying otherwise. And how long does shour sunscreen last then?