It's a fair question but not one easily answered. Far too many considerations to throw numbers out to the masses.
Let's consider the "risk-free" rate of return. We'll call this the rate the bank would give you in a cashable GIC with full CDIC protection. It's a little less than the rate of reported inflation and more so for real inflation and after taxes you'll lose a little purchasing power every year. Still, this is a bigger concern for someone at 40 than 80.
It's also important to recognize that a rate of return must be considered relative to inflation. If inflation is running at 6% and your return is 10% you are no further ahead than if inflation was 2% and your return was 6%. Real returns mean increased purchasing power.
We must also consider risk. Up to a point, taking on a slightly higher risk profile should give you a higher potential reward. The key word is potential. Taking on too much risk is a little like driving 110 mph on the highway. Better to give up potential rewards than lose capital betting on gambles. Achieving any targeted rate of return by taking on the least amount of risk should always be the goal.
Then one must consider the time frame. Rates of return over the last 12 months for an equity portfolio have been very robust. Should we consider this the norm? A term known as reversion to the mean implies that returns move back towards their historic long-term averages over time. In short, it's only meaningful to discuss potential rates of return for equity markets over periods of a minimum 5 and more likely 10 years.
There are many strategies designed to provide the illusion that making money in a passive investment portfolio is easy if you only knew what the "experts" knew. Most involve some form of leverage which does magnify gains as well as losses. It didn't work out so well for the "experts" at Bear Stearns which was sold for pennies on the dollar and later Lehman Brothers who declared bankruptcy after 138 years in business during GFC 2008.
At the end of the day the question is not necessarily what rate of return you could receive (although we'd all like more) but rather what rate of return do you need?