Comments from a (reformed?) data addict.
You might want to use 3% or 4% if you include dividends for a long-term 'real' (over inflation) US equity return.
Post-WWII, US equities median long-term real (over inflation) price performance (no dividends) has been about 1.3% to 1.5% using the Dow and depending on your time horizon (20yrs=1.4% 30yrs=1.6% 40yrs=1.3% etc). So add in something for dividends (yield since 1985 for the Dow has ranged from about 1.5% to 4.5%, with a quick visual guess of 2.7% or so for a median) for a more complete picture.
A piece of good news - once you reach a 32 year time horizon, it becomes highly likely that you will come out ahead of inflation (over 94% of the time, of course that is "only" 35 examples of 32-year time horizons eg 1945-1977, 1946-1978, etc).
some methodology notes...
I used CPI-U for inflation and the Dow Jones Industrial Average (30 stocks) since the data goes way back. In fact, 1896-present the long-term real price performance is more like 0.2% to 0.3%, not as exciting. If you start in 1931 instead (avoid the 1929 crash), you get very similar results to starting in 1945.
S&P 500 is my default current index, but price data on Yahoo for the S&P 500 only goes back to 1950. Since then, long-term real price performance (no dividends) is 2.1% to 2.5%. Yield for S&P since 1970 has ranged from just above zero to 3.1% or so, quick visual guess of 1% median). That's better price return than the Dow, mostly thanks to the S&P 500 including mid-cap stocks not just large cap. Total S&P real return of roughly 3% to 3.5% in line with Dow real return of roughly 4%...that mixes & matches some periods though.
Before CPI-U data starts, I used inflation estimates from westegg dot com/inflation/ . I inflation-adjusted index prices, and then looked at all x-year returns from 1 to 40 years and then the median of all data for any one length of returns (all 7 year periods, for example) to try to avoid a particular start date skewing the result. That's why I started with the Dow, as the S&P has just over 20 years of 40-year results (Dow goes way back, I started with 1896 annual data so 78 yrs of 40-year results). Got similar results whether looked at monthly or annual data (for a 10-year range that would be either 120 monthly data points or 10 annual data points). Yields I did NOT download, I'm trying to recover from my data addition
I looked at yields online in graph format at wsj dot com (I think the graphing requires subscription - yahoo dot com doesn't seem to have yield data for indices? or look for a total return index - and wsj yield data seems to reach back different number of years depending whether you've chosen annual or quarterly data or something)