If you're talking about the original X-Com, there's no hard time limit. The game gets more difficult over time, but you can never lose purely by playing too slo

If you're talking about the original X-Com, there's no hard time limit. The game gets more difficult over time, but you can never lose purely by playing too slow. Notably, the aliens never attempt to infiltrate Russia, so you can never lose by complete infiltration of the Council of Funding Nations.