Current technology allows roughly 9-month transits every 2 years or so when orbits align. You need heavy shielding when you leave the Earth's magnetosphere, but that's not a problem if you radically reduce the cost per kilogram of spaceflight (as I've said SpaceX is on the verge of doing). Also not as big a deal if you have propellant depots already in space. Trust me - the numbers have been worked out by smarter people than I.
But next technological steps are on the horizon that would allow for much more frequent trips with transit times of about five weeks: VASIMR (Variable Specific Impulsive Magnetoplasma Rocket). Testbeds already exist, and a scale demonstrator thruster will be tested on the space station next year if I'm not mistaken.
http://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket
For manned interplanetary travel, VASIMR would need a large space-rated nuclear reactor considerably larger than have yet been deployed, but not to any absurd degree. And certainly within range if this happens:
http://www.technologyreview.com/news/531836/does-lockheed-martin-really-have-a-breakthrough-fusion-machine/
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