No real time to let go..

There are times when we have to let people go, sometimes its people at work, sometimes friends and close relatives..
We can try, try real hard – at times never give up and be of help. There are just times when we have to let go, and let people find their way much as we’d like to help them. Can anyone define a lost cause? In my limited experience, there is no specific time.. its a feeling thing!