If by-elections are to result from the recent reports of election fraud in a growing number of ridings then a 30 day time limit might already be ticking.
The Canada Elections Act (see below) clearly stipulates that any application for a by-election resulting from election fraud must be filed within 30 days of when said fraud was known or should have been known.
At this time there is uncertainty about details and evidence relating to robocalls that attempted to suppress voting in a large number of ridings, but as broader facts are established, such as evidence that voter suppression robocalls were made, it would appear that an application to the courts should be made within a matter of days.
If no political party acts to file an application within the next week or so, citizens in robocall-affected ridings should begin the process on their own.
From the Canada Elections Act (PDF pg 328-329):
"If someone is representing themselves to be Elections Canada, giving false information, changing the polls, and the purpose is to confuse electors to the extent that you're attempting to discourage them from voting, then that is against the statute, in my view," said Kingsley. "This is not small potatoes because what you are trying to do is interfere with the right of Canadians to vote and that is a constitutional right in Canada."For a list of ridings possibly affected and other details see this Yahoo News article.