I'm building a deck and was wondering how far I could span 2x12 joists

Depends on the spacing, a 2 x 12 at 16" o.c. could span anywhere from 13 to 20 feet. Check this link:

Span Tables Floor Joists and Trusses | Raised Floor Living | Southern Pine Council™ | 1-504-443-4464 (http://www.raisedfloorliving.com/spantables-2.shtml)