Two things, really. Keep your key private, and stay within your plan's limits.
Your API key is tied to your subscription. Keep it private — anyone with it spends your quota, and abuse from a leaked key can get it revoked.
Each plan has a per-minute and per-day cap. Going over returns 429 Too Many Requests — slow down and it resets. Need more? Upgrade your plan.
One key is issued per Patreon membership. If you cancel, your key keeps working until the end of your current paid period, then lapses unless it renews.