How far can #12 wire be run without voltage drop becoming a problem?
25 feet is no big deal for voltage drop.  We start worrying about 70-100'.  

You don't need a 20A switch unless you're switching 20A of lights.  For a common household light, a regular switch will do.  

You're speaking precisely about wire type, i.e. NM-B, but that's absolutely the wrong wire for that job.  NM-B can't go outside.   I say "wire" instead of "cable" because you are better off using actual wires in conduit, which is to say THWN-2.  If you want to staple cable to walls at the ends of the conduit run, have the conduit end in a junction box, then use the junction box to splice between your NM (outside conduit) and THWN-2 (in conduit). 

You can get multiconductor cable similar to NM that is rated for outside, but it's more expensive than THWN and it will be much harder to cram down a conduit.  

Make sure you bury deep enough, 6" for rigid conduit, 18" for other conduit.