On the Free plan, meetings with three or more participants end automatically after 40 minutes; one-on-one meetings are unlimited. You can simply start a new meeting to continue. Pro and Business plans remove the 40-minute limit entirely, with meetings able to run up to 30 hours.
If your meetings keep ending at 40 minutes, check Billing → Plan to confirm which plan is active for the host — the limit is based on the host's plan, not the participants'.