Doyle and Keith have made good points. On late afternoon of July 1st, Confederate officers had no idea how many Federal troops occupied Culp's Hill or what condition they may have been in, nor did they know what Federal troops might be arriving on those heights by dusk. Not enough information to even guess.

Bear in mind that half of Early's Division had been deployed to counter a reported Federal advance towards Gettysburg on the York Pike. Of course that report was ill-founded, but it serves to demonstrate the problem of responding to information that may or may not be well founded.

Also, it's not a simple matter to move troops in column into position on unfamiliar ground and deploy them in line of battle. It requires time under any circumstances, particularly if soldiers are tired and columns are strung out along the road after a very long march, as Johnson's men were. As I recall, Johnson's men began arriving outside Gettysburg about 6:15 PM and were nearly all in position for deployment within an hour.

On July 1st the sun set about 7:30 PM, with diminishing light available for an hour or more afterwards.

As David suggests, if a substantial number of Confederates could have occupied and held Culp's Hill late that afternoon, the course of the battle would have been altered substantially.