ashaq,
I cannot give you an estimated amount of time, that is mainly determined by your particular setup. I can give some tips however.
1) do it first in a test environment to ensure that the system will work properly. This should give you an estimated time to actually do it in production.
2) If you have the resources, set up V8 DB2 server on new or different hardware and copy the data to it from the old server. This is simple and easy and if the upgrade does not work properly, you still have the old server intact to go back to.
3) Test the V8 DB2 client software with your environment. You may still need to use the V7 client with V8 server.
HTH
Andy