The restaurant reservations don't hinge on those with the dining plan, as it's open to everyone. People staying offsite (or live locally to WDW) can make a dining reservation at any time, as well.
The 180 day advance reservations recently changed, and as the previous poster stated, the reservations are now 90 days in advance. If you're staying at a Disney resort, the most convenient option is to call 90 days before your check in date, then make your dining reservations for the rest of your stay (up to 10 more days). It's a nice perk for Disney resort guests.