I got a sim to use while on holiday in Canada, I didn’t select auto top-up. So what happens after my plan expires?
How long can the sim be dormant? If I top up next time I visit (June 2019) will it work or will I need a new SIM?
Your service will stop completely and after 90 days the account and SIM card gets deleted and you need a new SIM card.
The SIM can stay dormant for a maximum 90 days. You could keep it going till next June by logging into your account every 90 and buying a $10 plan. That would let you hold onto the same number. But it's probably cheaper to let it go and buy a new one when you return to Canada.
Of course if you let the SIM card expire, and buy another one when you return, your phone number will be different. This may or may not be important to you.