So present boosters have marginal thrust to get one human to Mars alive. We can build boosters with several times more thrust, but we haven't done that yet, so we don't know the number of tries needed to get it highly reliable and safe. With bad management and/or not enough funds, it could take 20 years. Then we need to design the 2nd and third stage rockets. The return rocket might be thought of as a 4th stage. Lots of tests are needed to be sure these 4 stages work well and safely together, perhaps 20 more years. We can increase the safety, comfort, number of crew members, redundecy, options when a component failure occurs, and shorten the number of months in near zero gravity. We likely can rotate for artificial gravity, but each of these add mass at lift off, and increase the amount of testing needed. In theory, we could assemble the Mars craft at the ISS = international space station, but that compromises most of the uses of the ISS, and creates significant hazards for the assembly workers, and other ISS crew. It is doubtful that we could get all our parteners in the ISS to agree. Even with enthusiam, 40 years is optimistic and it is doubtfull the ISS will be safe 40 years from now, even with extensive renovations.
Ten billion dollars per year might not get us to Mars in 40 years. There is considerable uncertainty about hundreds of details. If we wait a few more years, we may have a space elevator, or other technology that reduces the mass, shortens the travel time, and we will surely have a better handle on some of the many details. Neil