Sessions can't tell him anything, since he's recused himself. As far as the date goes, it's not possible to determine, since much of what is being pursued is relative. The people connected have been charged with various offenses to be used as leverage in their cooperation. The investigators have no control over trial dates and lengths. There have also been ongoing negotiations as to whether or how Trump would answer questions from Mueller. Setting a date would mean potential witnesses or targets of the investigation need only wait until the clock runs out. In that way, it's exactly the same as military policy timelines in any theater. It's not as big of a hill as you think.
Conversely, the investigation isn't a political sword of Damocles for dragging out till the election in the hope the Dems take Congress, no more than a military action should be continued for poltical purposes. Not a big hill to climb there either.
It also doesn't mean that Sessions can't honestly sunset the investigation if something isn't found. Once trials and interviews are done, and if nothing comes out if it, then it's absolutely appropriate for the DOJ to demand an end. Sessions recusal in fact reinforces that decision, it does not work against it.
