Jhilkaydai,

If you process you application through premium process, you will know about decision within two weeks, technically. If for some reason, you don't, then surely within a month. If you go through regular process, it might take two months or up to three months you know about the result.

When you're on F1 going to college after your OPT expires, you can easily cange your status to H1B after Oct 1st. As far as I have seen, there is no hassle for this process.

You do not have to go out of the country just to change your status from F1 to H1B holder. But you do have to go out of country to obtain a new visa if you are travelling to other country. Many people with H1B's go to Mexico and Canada to get stampped a new visa to be able to return to the US after travelling to Nepal, India, or wherever they want. So, if you plan on going to Nepal, you have to go to US consulate/Embassy in Mexico, Canada, or Nepal to get a visa, otherwise, you do not have to leave the country just to change your status.

Hope this helps. All the Best.