Read about it at
http://www.cic.gc.ca/english/information/faq/work/index.asp#lowThe maximum time you can get is 24 months. Then you have to leave for 4 months. Then you can get a new work permit for another 24. Doesn't matter if you change exmployers, 24 months is still the maximum on a low skill permit.
Normally itīs not possible to apply for a permanent residency under low skill. There are a few jobs in AB and BC that qualify to apply under provincial nominee program with help of employer or if you qualify to apply as a skilled worker, you can do that after 1 year in Canada.
You can also try to get a skilled job or get the employer to promote you. If you got a skilled job, you don't have to leave after 24 months and have more options to apply for PR.