Status is annual, except that some airlines grant lifetime status to people who hit the million mile level.

It varies quite a bit by airline, but most allow their elites to select exit rows in advance, so none are available by the time they open up to non-elites. Some charge a fee for exit rows and other premium seats to non-elites, so you can only get them for free if all of the non-premium seats are full, and only premium seats are left. Typically, those assignments are made at the airport, not at the 24 hour online-checkin window. Again, this varies by airline.

One thing to keep in mind is that many of the elites who reserve exit rows seats are on the upgrade list to first class. Many of these upgrades don't clear until boarding time, or shortly before, so asking at the gate if an exit row seat has opened up can help, again assuming that all non-premium seats are full and the agent needs to put someone there anyway.