My preference is right after blocking rehearsals are completed--usually 2 weeks in.  By that time they are usually familiar enough with each scene to walk through it once with the script in hand and then drop the script with a stage manager prompting.  Usually some of the scenes later in the play need another week for them to get comfortable with, but generally we're able to work each scene without a script in hand before we can do Act Runs.  If we're lucky, we'll even get a few polishing rehearsals in before runs.