Most people do the hrs building after ATPLs, but there's no reason why you cannot do it during, or in between the ATPLs. Although I did most of my hr building directly before the CPL and IR, I always quite fancied the idea of taking a months break between the first and second phase of the ATPLs to do hr building, and then having another hr building session before the start of the CPL/IR to get me back in the swing of things.
Since I was working full-time this was not a viable option, but it certainly would have helped general motivation.