I only subscribe to the ones I know I'll read.
Over 100 on my list, i look at content mostly (like how well written it is) the decide to sub or not.
For longer stories that have over 100 episodes, I tend to judge it by art because unless it has something unique, there is no way I'm going to sit and read 200 long pages. Thats not to say I ignore it completely. I still sub if the content is something I like, but i put it off for reading when i have time. Right now, out of all the series I'm subbed to, only 2 I have yet to read and both are long stories