Technically you can re-apply the next day after you receive a refusal. But the most important thing to do before you re-apply is to figure out exactly why you were refused.
Those who receive just a refusal letter get very little information. In many cases the visa officer uses a simple form and ticks off one or two generic answers. It is quite rare for an officer to provide any meaningful explanation.
If you were interviewed and the officer explained to you why YOU were refused with some meaningful explanation then, obviously, when you reapply you will have to address that problem, and address it directly. Remember that visa officers (like all decision makers) want you to corroborate what you say in your application, so it is best if you have documentary proof of your claims.
If the officer did not give you a meaningful explanation, but just gave you a refusal form with a tick next to one or two generic explanations, then you will want to get a copy of the visa officer's notes. Visa officers do make notes when they make a decision and, as informal as those notes can be, those notes are their official reasons for their decision.
Once you review those notes you will have a better idea why the application was refused and will be better able to address that issue in a new application.
Gregory James
Canadian Immigration Lawyer
gjames @ GregoryJamesLaw.com