Can I use an extension cord for a window AC unit?

No, skip the extension cord entirely for window ACs. Extension cords cause voltage drop and heat buildup, and most aren’t rated for continuous high current — especially during AC startup. Even a heavy-duty 12-gauge cord over 25 feet is risky for a 15-amp unit. If you absolutely must, use a 12-gauge or thicker cord, 6 feet max, but a dedicated outlet is the only safe bet.