IMHO, where you live (the airport(s) you can use) makes a lot of difference.
For MSP it usually isn't a good idea to "wait" for a sale. I usually start looking as soon as my lodging is booked (11 months in advance). As soon as I see a resonable price come up, I jump on it. Lately, that has been anything less than $250 RT for non-stop flights (NWA) and $160 RT for one connection (which can occasionally be found on Delta).
If you have no idea what "reasonable" prices are for your airport of choice, post the airport(s) and I'm sure you'll get several replies. Good luck and happy hunting!