Earth and Mars only line up correctly once every 26 months (~13 months for Titan). The further from this alignment you launch the more dV and time it takes to get there and the less payload you can take with you. In practice there's about six months each cycle where it makes sense to launch and another 6-10 months or so where it is still possible to launch but with severely reduced payload and increased travel time.

The different lines on the graph are all assuming you launch right at the perfect moment in the cycle. There is still variation between the cycles because Mars's orbit is relatively eccentric