Tampon Problems

Hey everyone. I’m 15 and new to using tampons instead of pads. I never keep a tampon in longer than I’m supposed to. If I’m going to the bathroom and I look down and see the string is red then I know its time to take it out and change it.

I just took out a tampon with a red string and the tampon itself was mostly white except for at the bottom..

How am I supposed to know when its actually full ??

Ps: sorry if this doesn’t really make sense I’m trying here