Canada usually grants multi-entry visitor visas up to passport expiry (up to 10 years) from the date of issue.
For single-entry visas, they are usually valid from up to six months before the expected date of travel until one months after the expected date.
Nigerian citizens face significant challenges when apply for Canadian visitor visas as the country is considered high-risk for overstays and asylum claims. Many people use international conferences held in Canada as a way to get to Canada to claim asylum. Of course, refugees are expected to use any feasible means to get to safety, but the government does try to prevent "abuse" of visitor visas.
For example, during the International AIDS conference in 2022, almost one in six visa-required attendees claimed asylum in Canada, and this is after the IRCC had rejected many visa applications from high-risk countries, including Nigeria:
Ottawa rejected 83.5 per cent of visa applications by prospective conference attendees from Nepal; 55.8 per cent of those from Nigeria; 53.6 per cent from Pakistan and more than 40 per cent from Cameroon, the Democratic Republic of Congo, Ethiopia and Ghana.
https://www.cbc.ca/news/politics/aids-summit-montreal-visas-refugees-1.6915326
Justly or not, you should expect the high possibility that your visa may be rejected, or at least take a very long time for processing. You should prepare and submit the application well in advance, with as much good evidence of why you would return to Nigeria as you can provide.