Hi Ezz,
My boy is close to 5 and a half months and he still only sleeps 30 mins. I know he can settle himself as he has done this a couple of times, but he seems to choose when and where he does it. Most days it takes me an hour to get him to sleep through the kicking and screaming and then he only sleeps for 30-35 mins. It is a rare treat when I can re-settle him back to sleep, but it takes a lot of effort. I am so sick of people telling me to let him cry it out and it will solve all my problems. It gets me down most days as i am house bound cause he is grumpy all day due to his lack of sleep. I have read nearly every book under the sun, but nothing has helped... I do understand where you are coming from.....it is SO frustrating...
To answer your question, I think the hungry sign is when they dont stop crying and it just gets louder and louder till they get food...