Sandoval, 23, burst onto the scene with a late season cameo in 2008, then finished seventh in the 2009 NL MVP voting when he hit .330/.387/.556 with 25 homers in his first full season. He won't be eligible for arbitration until after the 2011 season, but that doesn't mean San Francisco can't secure some cost certainty now.
The best comparable for a contract extension might be David Wright, who inked a six year, $55MM deal in the middle of his second full season. The Mets' third baseman hit .306/.388/.523 with 27 homers in his first full season, and was hitting .308/.382/.545 on the August 2006 day he signed his deal. Sandoval started the 2010 season at nearly the same exact age that Wright started the 2006 season, but the biggest difference between the two is that Wright was a far safer bet to remain at third base long-term.
With a 5-foot-11, 245 lb. frame (that's what the team's official site says), Sandoval has a -2.3 career UZR at the hot corner, and is expected to slide over to first as he gets older and potentially gets even bigger. That's something the Giants will have to consider if they approach their best hitter with a contract, that it's easier to find a first baseman that hits like he does than a third baseman.
Wright's deal bought out his last remaining pre-arbitration year (for $1MM), his three arbitration years (for $22.5MM total), and two free agent years (for $29MM total). There's also a $16MM club option ($1MM buyout) for the 2013 season, which would have been Wright's third free agent year. That framework seems reasonable for Sandoval, though the Giants might not want to assume so much risk with a bad bodied player. Buying out his last four years of team control for $23MM or so with (ideally) a club option or two for some free agent years would make sense.