It took about 6 years for my mother in law to get a visa, but she applied for permanent residence, not for a visitor's visa. 4.5 of those years were issues with with mexican side. once that was straightened out, the US side went fairly smooth.
in your situation, you could have appealed the decision for a smaller fee than starting from scratch. also, apply for permanent residence instead. sometimes they are more easily granted, especially if you can prove that she will not be a burden to the government (meaning you can support and house her without needing financial assistance).
a visitor's visa will require her to prove she has business ties in mexico that would make her lose income if she stayed in the US. owning a home and bank account doesn't show that.
applying for PR over visitors seems counter intuitive. That's our gov't in action. She just wants to be able to visit my wife and kid.