I would say they are pretty good, but if you live in a area that use salt on the roads to melt ice, you better wash the undercarriage frequently to avoid corrosion. 

As far as I know current Toyota frames are not as susceptible to severe/fatal rust problems as the infamous Tacoma debacle of the mid-2000s. Even new ones though will of course start to rust if exposed to a yearly salt bath for a long enough time.

I've found that spraying the underside of my winter-use vehicles with Fluid Film each fall to be effective at retarding winter corrosion. I use an inexpensive air-powered spray gun. (There are a number of similar products on the market.)

Along with @chucktobias I have used fluid film, although I saw some tests run with crc marine heavy duty corrosion inhibitor, and the tests showed that it stuck on and provided better protection overall. The fluid film was better in cracks and places that you want it to seep into, because the crc does not much move once you put it on. It also hardens like a thick wax.

I am in the process right now, I just took a break, from painting and then hitting the frame with crc and fluid film of the tight spots. Better safe than sorry.

I own a 2005 Tacoma and was lucky enough to have the frame replaced but only after Toyota was forced to replace it due to losing a class action suit.  Many others who own other Toyota/Lexus SUV and truck owners have not been so lucky and are stuck with unsafe  vehicles.  A friend has a Lexus SUV that has been declared "unsafe to drive" by his Lexus dealer due to frame rust, lucky him.  Its not just Tacomas.

While my Tacoma has otherwise been a good truck, I would NEVER, EVER buy another Toyota/Lexus Suv/Truck again. I still have permanent deep rust stain on my driveway in the outline of the Tacoma as  a reminder.