How long did the bombing of a Toronto–Montreal–London–Delhi route of Air India have an effect on Canada-India relations for?
Canada–India relations:  Most notably, the bombing of Air India Flight 182 by Canadian residents, killing a vast quantity of Canadian citizens, had effects on relations for about 20 years. Air India Flight 182: Air India Flight 182 was an Air India flight operating on the Toronto–Montreal–London–Delhi route.
about 20 years