You can use indefinitely. While it doesn't include real-time protection, you can manually scan your system whenever you suspect an infection. For many users with common sense and a good primary antivirus, this free scanner is an excellent second opinion tool.
: Search for the confirmation email received at the time of purchase from Malwarebytes or an authorized reseller. You can use indefinitely
Modern Windows Defender is actually excellent. Combined with Malwarebytes Free (manual scans), you have robust protection without paying. You can use indefinitely
— Malwarebytes offers a legitimate free version and paid premium options that support ongoing development of their security tools. Bypassing payment undermines their work. You can use indefinitely