So I did some noodling around in my transfer window and resonant orbit worksheets, and here's the breakdown: The usual way, doing a Hohman transfer during the first K-D window (day 225), waiting for the next D-K window (day 1062), and then heading home (~318 days), will get you back to the KSC around 1,155 days after launch. Interestingly, the Hohman transfer orbit to Duna is also almost exactly 3:2 Kerbin resonant, so if you did this as a flyby you would end up back at Kerbin 3 years after launch, or 1,278 days later. You could however shave almost a year off of this by launching to a 2:1-resonant orbit that encounters Duna either on the way out or on the way back in. Not sure how close to the window your game is now, but you would probably have to do this at least a few weeks beforehand, actually you do it after!, h/t @Kryxal). Anyway, that would get you back to Kerbin in 852 days, which is roughly the fastest you can do it without shredding the pork chop with excessive dV. Hope that helps!
PS: just for due diligence, I'll say that you could shave a few more weeks off of this by doing a retrograde DSM near your AP, that would lower your PE and cause you to re-encounter Kerbin somewhat sooner. But the price for those 4-5 weeks would be a very high-speed Kerbin re-entry, which may not be worth building for if you're trying to save money.
herbal space program's post in Gravitational slingshots... any advice? was marked as the answer