Question: After compiling if i make any change in kernel source code . What should i do , compile kernel from starting by cleaning "make clean" and running make or can run make directly. I am facing difficulty because make takes around 2 hr to complete. Every time it is difficult to wait for 2 hrs .

Answer: 
Just running `make` will rebuild only the parts that have changed. You need `make clean` if you change anything major (like 64/32 bit architecture, etc.) And if you are just changing modules, you can just `make modules` to rebuild only the modules that have changed.
In almost all cases calling just `make`, without cleaning will be working OK and this is exactly what you should do. This will only recompile files that were changed and some dependent files, if needed.

There are occasional errors in kernel makefiles which led to wrong dependencies between files which in turn may sometimes led to some parts of the kernel not being recompiled, even if they should. So in case of some strange behavior, you may want to try to do clean rebuild of the tree.

Also note that you may speedup your kernel compilation by using `-jX` (where X is a number of concurrent compile jobs), especially on multiprocessor CPUs. You may also want to use `ccache`.