I wonder if there's some calculator or math way to calculate it? I have couple ships in Kerbin orbit waiting to be shot towards, Duna, Eve and Joolian system. But they are mostly probes, as I never yet been outside Kerbin and it's moons. I'd like to see how my present design are doing in interplanetary journey before attempting crewed missions. And I wonder how long would I have to wait for next launch window? If for example after arriving at Jool I find that I don't have enough fuel for return trip I will send tanker, but if waiting for correct planet alignment will take years poor Kerbals will be bored to death so far from home...