The slow down time, if Gs remain constant, will be another 10 years. Both the acceleration and slow down phases will be less than 20 years actual to the crew, however, they will take at least 20 years their time to arrive.
Then there is the slight problem (pointed out by a 10 year old as I recall) that it will take near infinite mass of propellant to achieve light speed, since mass increases as speed increases.
So it's back to the drawing board, unless a direct wormhole system a la Stargate, or an existing mode is made available a la The Culture (were effector fields solve the unwanted energy problem) which uses ships that gain traction above and below the energy grid (Plank Space?) and travel at speeds measured in hundreds of kilolights per second.