LITS is probably a good example of a time frame we could expect a full tour announcement if it's to happen:

On March 27, 2008, after much speculation, Nine Inch Nails announced a series of North American tour dates. A statement from Trent Reznor accompanied the announcement:
Lots going on in the world of nine inch nails...

Confirming a very poorly kept secret, the band has been reformed (details coming!) and we are about to embark on an extensive tour. The first wave of dates I can mention at this time appear below. Many more to come!
See you soon."

Tour Dates
Date/LocationVenue
2008/07/19 Los Angeles, CAThe Forum (Dress rehearsal)
2008/07/25 Pemberton, BCPemberton Festival
2008/07/26 Seattle, WAKey Arena at Seattle Center
2008/07/28 Edmonton, ABRexall Place
2008/07/29 Calgary, ABPengrowth Saddledome
2008/07/31 Winnipeg, MBMTS Centre
2008/08/02 Minneapolis, MNTarget Center (POSTPONED TO NOVEMBER 25, 2008)
2008/08/03 Chicago, ILGrant Park (Lollapalooza)
2008/08/05 Toronto, ONAir Canada Centre
2008/08/07 Uncasville, CTMohegan Sun Arena
2008/08/08 Worcester, MADCU Center (POSTPONED TO NOVEMBER 9, 2008)
2008/08/10 Baltimore, MDPimlico Race Course (Virgin Mobile Festival)
2008/08/12 Knoxville, TNKnoxville Civic Coliseum
2008/08/13 Duluth, GAGwinnett Arena
2008/08/15 Oklahoma City, OKFord Center
2008/08/16 Houston, TXToyota Center
2008/08/18 Dallas, TXAmerican Airlines Center
2008/08/20 St. Louis, MOScottrade Center
2008/08/22 Cleveland, OHQuicken Loans Arena
2008/08/23 Auburn Hills, MIPalace of Auburn Hills
2008/08/25 Toledo, OHSeagate Convention Center
2008/08/27 East Rutherford, NJIzod Center
2008/08/29 Philadelphia, PAWachovia Center
2008/08/31 Lexington, KYRupp Arena
2008/09/02 Morrison, CORed Rocks Amphitheater
2008/09/03 Salt Lake City, UTThe E Center of West Valley
2008/09/05 Oakland, CAOracle Arena (formerly Oakland Arena)
2008/09/06 Inglewood, CAThe Forum
If we don't hear anything else by the second week of April I'm buying my ticket to NYC.