There shouldn't be an exact moment you switch to a conservative portfolio. As you get nearer to retirement, you gradually rebalance. Often this is as easy as simply placing new contributions in safer investments. Less often it just involves exchanging one set of funds for another, say once per year.
Again, Target Retirement funds handle this completely transparently for you and require literally zero hands-on involvement.
This is quite simply nowhere near as hard as you're making it out to be.