Work Permit - 2

[shlomo]

The paragraph reads:

“These steps must be followed before you apply for a work permit:
1.An employer must first offer you a job.
2.HRSDC must normally provide a labour market opinion or "confirmation" of your job offer. However, some types of work are exempt from this process.
3.After HRSDC confirms that a foreign national may fill the job, you apply to CIC for your work permit.
To get a work permit, you must fill out an Application for Work Permit. A work permit is issued if the application is approved.”

I am still not clear about this.

The paragraph is not specific about whether you can apply for work while in Canada on temporary visa or not.
Can I be in Canada, apply for work and then once I get the offer apply for the Work Permit? How long does the procedure last?
Once I got Work Permit, how long does that permit last? Is it linked to a job? Can I change the job and still be using the same permit?

Thanks for your help,