WhatsApp accounts are attached to the phone number, not the SIM. In my experience, WhatsApp continues to function until WhatsApp needs to verify the account; WhatsApp then sends a verification code to the attached phone number. Typically, WhatsApp asks me to verify my account whenever I reinstall WhatsApp or if it detects that I have configured WhatsApp with the same phone number on multiple devices simultaneously.

So you can continue to use your WhatsApp on the same device indefinitely, regardless of the phone number of the SIM card that is installed. However, if WhatsApp needs to verify your account for any reason, then you are stuck.