1) You will get your OPT within 3 months after they give your a receipt ( once they receive your application, they will give you a receipt). If you did not get OPT within 3 months and you found a job, you could get a temproray OPT at the end of the 3rd month. Another situation is that if you have offer BEFORE Opt application, by submitting a copy of this offer letter with your OPT application, the OPT issueing process will speed up, meaning that you can get your OPT more quickly.

2) officially speaking, your potential job should be directly related to your major. While your employer might not know this, so they might hire you even though your job is not relating to your major. However, you will be responsible for all the possible conesequence. I do not think it worth the risk.

I hope my answer is helpful.

thanks so much for your reply. It really helps.

but what's the meaning of

However, you will be responsible for all the possible conesequence. I do not think it worth the risk

do you know what kind of conosequence i'll get? i can't change status from F1-H1B?

anyway, thanks.