As per rule of 72, [if S&P interest rate is 10%](https://www.nerdwallet.com/article/investing/average-stock-market-return), money should double in 7.2 years.

But there are other websites which show different numbers, [such as this one says 9.2 years.](https://www.ubs.com/global/en/wealth-management/chief-investment-office/market-insights/house-view/daily/2019/doubling-time-and-the-rule.html)

I understand inflation has a role to play in this. But I am trying to compare S&P against other investment options such as real estate for example, so I am not interested in factors like inflation since they will be constant across these options.

So if I simply buy a S&P ETF, every 'how many years' can I expect it to double (on an average)?

Thanks.

Edit:
I plan to put away the money for long term and forget about it (reinvesting dividends and any other returns there might be). In this scenario what would be the 'average rate of return'.