Here's a brief look at the history of teams relegated from the Primera and how quickly they made it back to the top flight, if they did.
2004-05: Levante: won promotion back immediately; Numancia--promoted in third year; Albacete--have not returned
2005-06: Alaves--have not returned; Cadiz--have not returned; Malaga--promoted in second year
2006-07: Celta Vigo--have not returned; Real Sociedad--returned in second year (finished fourth in 2007-08); Nastic--have not returned
2007-08: Real Zaragoza--won promotion back immediately; Murcia--have not returned; Levante--won promotion back in second year
2008-09: Real Betis--won promotion in second year (finished fourth the year before); Numancia--have not returned; Recreativo--have not returned
2009-10: Valladolid, Tenerife, Xerez: none have yet returned
2010-11: Depor, Almeria, Hercules: Depor certain to return at first attempt; Hercules likely to make playoffs; Almeria not as likely.
What I take away from that is that the "larger teams" in terms of budgets--La Real, Depor, Zaragoza--have had good success in escaping the Segunda 'purgatory', but smaller clubs have not. Of course, often relegation from the Primera has coincided with a financial crisis in the club; that was true of Celta (who are probably the 'largest team' not to make it back for a while).
Overall, though, the odds of a first-time return aren't as high as one would expect: only three teams, Levante, Depor, and Zaragoza have succeeded at the first try, and four other teams got back in the second year. 12 either haven't made it back at all or took longer than two years to do so, and we don't know yet about Almeria and Hercules.
The promotion playoffs are a fairly new thing, and Sociedad or Betis might have returned in the first year if they had been in effect. It is interesting that two teams have not been promoted in the first attempt in the same year since Sevilla and Betis, both relegated in 1999-2000, promoted in 2000-2001. Doesn't bode well for both Sporting and Villarreal returning (Racing has huge financial problems and might not even remain in the Segunda the way things are going).