For WinRar, how long should a password be to make up for a lack of complexity? Assuming that the best alternative to complexity is length, how long do you recommend a password or phrase to be when using WinRar?

Note that I'm aware that it's not always that simple, but I would like to get this right.
how long do you recommend a password or phrase to be when using
  WinRar?


This very much depends on how you choose your passwords. The "right" answer can be anything from 11 characters to 200 characters.

So assuming fgrieu's research is correct (it usually is), the PBKDF uses at least $2^{15}$ rounds of hashing, giving us effectively 15 bits of security "for free". Now with the target being 100-bit security (which is reasonably secure for most intents and purposes), this leaves 85-bits to go. Now assuming you use the full ASCII-Range and good cryptographic randomness, this is about 11 bytes, or 11 characters. However assuming humans being humans, you will get much less randomness from short passwords. A conservative estimate would be 40 characters (with a bit more than 2 bits of entropy per character) and a more aggressive estimate would be 16 characters (assuming nearly fully random pick from upper- and lower-case latin letters).