The super-digested version is that it's highly unlikely that humans could survive for very long outside our magnetosphere, given exposure to radiation level (the longest time is only 12 days).

On re-reading my post, I need to clarify this sentence.

The longest time that a human has spent outside the magnetosphere is 12 days. That is not the longest amount of time a human could survive outside the magnetosphere - which on re-reading, might be what someone might take from my post.

The real problem is radiation. We've got good data on long-term exposure to low/zero gravity - people have spent a lot of time in earth orbit, and we have a fair amount of information on what it does to our biology. But because humans have never spent much time outside the magnetosphere, we don't know for sure how long a person could survive - we don't have a lot of data. Because once you get outside the magnetosphere, you're no longer protected from radiation - both from the sun and from cosmic rays. The latter are particularly difficult to deal with, because they're such high-energy particles that you can't really shield against them (as your shield gets thicker and blocks more particles, you actually end up increasing the secondary radiation exposure from the energies released when the shields interact with the cosmic rays).

One study gives a max time in space at around four years - which would make a round-trip to Mars theoretically possible (you'd still up your lifetime risk of cancer somewhat with the 18 months in space, though).

science.org - Humans return moon scientists confront dangers deep space radiation
newsroom.ucla.edu - Safe for humans fly to mars