Let's start with the basic kinematics equation:
s-s0 = v0*t+0.5*a*t^2
s-s0 is change in position
v0 is initial velocity
a is acceleration
t is time
let's just set s0 and v0 equal to 0
solving for t, we get:
s in this case is half the distance from earth to Alpha Centauri, which is 4.36 ly away, or 4.1248*10^16 m
Therefore, at 10 m/s^2, it will take 6.422*10^7 s, or about 2 years, to get half way.
So, you're looking at 4 years to get there.
Oh yeah, one little problem: you'll be going twice the speed of light when you start your deceleration.