Some investors aspire to build a seven-figure Stocks and Shares ISA portfolio. The good news is that many have already trailblazed a path to ISA millionaire status.

Clearly, these people were committed to the cause, investing regularly, come rain or shine. But could I get there by investing £20k then doing nothing? Well, the answer is yes, depending on my investing time horizon.

£20k to £1m

Historically, the S&P 500 index has generated an average annual return of 10.7%, including dividends. And while UK shares have averaged a bit less, there’s nothing stopping investors from putting money to work in both markets.

Indeed, just over half my portfolio is invested in high-quality US stocks like Shopify, The Trade Desk, Mastercard, Axon Enterprise, and Intuitive Surgical. The rest is in UK shares.

Therefore, I think it’s entirely realistic to aim for an average 10% return long run. Were I to achieve this, a single £20,000 investment — thanks to time and compounding — would turn into £216,694 after 25 years. That’s with dividends reinvested rather than spent.

Not bad, but clearly still a long way from a million. To reach this, it would actually take just over 41 years.

So, if I was 21 (unfortunately, I’m not!), this means that I could have a million-pound portfolio a decade or so before the official retirement age. Nice.

Remember though, past performance is no guide to the future. Average returns could be lower (or higher). Plus, there’s inflation to factor in, which would reduce my future spending power.

An example of a FTSE 100 share that I think could contribute towards this goal is Smith & Nephew (LSE:SN).

The company was founded in 1856 by Thomas James Smith and originally refined cod liver oil. His nephew joined in 1896, leading to the name. Today, Smith & Nephew is a global medical firm that specialises in joint replacement technology.

The stock is currently 46% lower than its all-time high set back in 2019. The coronavirus hit the firm’s profits hard as operations ground to a halt worldwide. So another pandemic is a risk.

Also, GLP-1 weight-loss drugs like Wegovy have been weighing on the share price. The uncertainty is related to whether fewer obese people will reduce demand for joint replacement surgery.

This seems unlikely to me. In fact, by aiding weight loss and potentially reducing inflammation, GLP-1 drugs could make more patients eligible for surgery. So these drugs could actually expand the market!

Further, the number of people aged 60 years and older is set to rise to 1.4bn by 2030 and 2.1bn by 2050. A rapidly ageing global population plays right into the company’s strengths.

The stock is trading on a forward price-to-earnings (P/E) ratio of just 14.9. That looks cheap and there’s also a 2.9% dividend yield. I’m watching it like a hawk.

Making a million quicker

If I invested £500 each month, adding to my initial £20k, it would take approximately 27 years to get to £1m with a 10% return.

But what if I contribute £1,000 a month? Fantastic, because that would knock off about five years!

Finally, if I could somehow max out the £20k ISA limit each year, I’d need just under 18 years to grow my ISA to millionaire status.