Missions to mars are feasible only once every two years?

Yes.

Explanation:

If you want to use the least fuel, then missions to Mars are feasible every two years.

You want to launch when Earth and Mars are close together, and this happens about every 26 months.

However, Earth travels faster than Mars, so the spaceship must launch when Mars is 44° ahead of Earth in its orbit.

The ship will then use the most efficient trajectory to arrive at Mars.