In my FASA/Realism Overhaul career, I missed my Duna transfer window by about 70 days. So, naturally, I decided to see how fast I could get to Duna using chemical rockets without the alt-f12 menu. A massive heat shield was installed, and the probe was launched on board a modified Saturn 1B.
36 short days later, the Duna Orbiter entered the red planet's sphere of influence moving at 10km/s relative to Duna. The moment the spacecraft touched the wispy outer edges of the atmosphere, a decoupler failed and the heat shield rammed through the rest of the spacecraft.
Long story short, just wait for the next launch window.