Planning horizons for ATCOs can be pretty far ahead, depending upon what's going on in your sector. From my experience (not an ATCO though) controllers will generally try to anticipate the impact of their instructions on downstream sectors. Likewise, if their plan will go easier if a preceding sector does something different with their traffic, they will pick up the telephone and co-ordinate.
An ATCO always wants to be ahead of the traffic (proactive not reactive) so will plan as far forward as they are comfortable. This often means they are including traffic in other sectors in their scanning and planning, as well as actively working to keep them apart and moving most efficiently in their own sector.