Dual Momentum* investors working with three ETFs plus SHY as the cutoff or "circuit breaker" are recommended to remain 100% invested in U.S. Equities (VTI). Check out the screen shot below.
The look-back period is 252 trading days or one year. The following table comes from a spreadsheet known as the Kipling Tranche 1.7 and more information can be found by clicking on this link.
Swensen Six Recommendations: Investors who prefer more diversification than is available from three ETFs, should take a look at the "Swensen Six" and apply the Tranche Model. The Tranche Model is designed to reduce the problem of "luck-of-review-day" that plagues most investors who update and rebalance their portfolio on a regular basis. Without going into a lot of detail, the Tranche Model recommends investing in four (4) ETFs at this time. The data is accurate as of 10/15/2015.
Following the model below, an investor would invest in SHY, VNQ, TLT, and TIP as shown in the Required column. I would round the purchases to the nearest 50 shares.
In this second model, the look-back periods are 60 trading days with a 50% weight, 100 trading days with a 30% weight, and a 20% weight assigned to 14 trading days volatility. A maximum of two (2) ETFs are assigned to each portfolio and I am using eight (8) portfolio offsets.
* Dual Momentum is a trademark of Gary Antonacci.