External crack

Put tape around the crack before you apply the glue, remove surplus glue with a damp cloth before it goes off!! It will end up a lot worse if you don't do it!!

bagavan said:

I am a bit scared to apply glue...As it might cause some other complication..

Take a single edge razor blade and insert just the very tip, just enough to open the crack the tiniest bit then squeeze a very small drop into the opening.
apply pressure to hold them together for a few seconds like 30 then carefully move the lid ope and shut to make sure it only has stayed in the crack and you didn't squeeze enough into the opening that it dripped onto the post that allows it to open and close.

It sounds tricky but it's not that really a big deal use a very small sewing needle to get the smallest opening in the tube of super glue and before you apply it try squeezing a few drops onto another throw away surface to get a feel for how much is going to come out.
Left un-repaired it will eventually need replacement
Fabe

bagavan said:

Then how can I apply the glue?u have seen that the hinge is not completely cracked..

Then by following my previous driection will work in you favor open up the very end with a single edge razor blade jut the corner or and exacto knife and apply the glue.

Trust me it's not completely crcked but well on it's way and will eventually be if you don't take some measures to stop it's growth.
Fabe

bagavan said:

how many hours should I leave it to dry up?

Depends on what glue you use? Just leave the lid open 24hr after you have done it!! Should be ok!!

bagavan said:

how many hours should I leave it to dry up?

If you have applied it correctly super glue only takes a little while to dry. So as apprehensive as you are about fixing it I would let it dry at least an 1/2 hour or more. Just remeber you can't move the top or cause it to try and reopen while it's trying to dry.
Fabe

Good Man!!