The shorter the timeframe that the money is needed, the higher the allocation to bonds. Vanguard has (in Canada) a bunch of 'all-in-one' ETFs that have as their holdings other ETFs in various asset groups: Canadian equity, US equity, international equity, bonds.
Shortly after they were announced, someone back-tested their returns to determine which of the offerings a person should get:
> I analyzed hypothetical Vanguard asset allocation ETF performance over the past 20 years ending June 2019, and here’s what I found:
> The worst 1- and 2-year periods were negative for all five ETFs.* [i.e., put it in term deposit to at least try to keep up with inflation]
> The worst 3-year period was negative for all ETFs except the Conservative Income ETF Portfolio (VCIP), which holds 80% in bonds; even still, VCIP only returned 1%.
> The worst 4-year period was negative for all ETFs except VCIP and the Conservative ETF Portfolio (VCNS) [60% bonds]. But these only returned 2.2% and 0.2% respectively.
> Looking further out:
> If you need the cash in 5–9 years, VCIP or VCNS should be the only Vanguard asset allocation ETFs on your radar. Even the Balanced ETF Portfolio (VBAL) [40% bonds], which allocates 60% to stocks, returned only 0.3% over its worst 9-year period.
> If you won’t need the cash for 10–14 years, VBAL could be an appropriate choice, as even its worst 10-year return during this period was around 2%.
> If you don’t need the cash for 15–19 years, you could look at a more aggressive ETF, like the Growth ETF Portfolio (VGRO). [20% bonds]
> If you’re investing for 20 years or more (and you are comfortable dialing up your portfolio risk to eleven), the All-Equity ETF Portfolio (VEQT) might be right up your alley. [0% bonds, 100% equities]
* https://www.canadianportfoliomanagerblog.com/choosing-your-i...
Note: if you're looking at long time-horizons (e.g., retirement in 20 years), and you can take higher risks, does not mean you have to: you need to first determine what your goal is (e.g., how big of a pot of money you need), and then work backwards from there to determine what kind of returns are needed to get there. If you need 'only' 3% returns, it probably is not necessary to take on extra risk to chase after 6%.