Well, this is the thing, and one variable is dependent on another.
The most important thing is how much you save. This is probably more important than the rate of growth in most cases.
Why? Because your savings rate is inversely proportional to your spending rate. If you spend everything and never save anything then you will never have anything contributed to any investments and have nothing to sustain your spending if you ever stop working, so you can never retire . If you only save 10% of your income then you are spending the other 90% At that rate you need to work for 50-60 years to sustain that level of spending. If you double this to 20% then you are not only saving more, but also crucially spending less of your earnings, so you can expect to grow your investments to a level that can sustain your level of spending after 35-40 years. If you can achieve a 33% saving rate then you can be in a position to retire after 25 years, save 50% and you can retire in 17-20 years... see where this is going?
Savings is hard if you don't make a sizeable income. For some people it is hard even if they do make a sizeable income, but the simple fact is that most people don't save enough. There are people who do manage to save a big amount of their income, even on fairly modest salaries.. it's about lifestyle choices. But nobody is going to do it for you, or force you to do it, and those who don't save enough will talk about how its impossible for them to save more instead of figuring out a way to do it.
The other discussion to have is the generosity of the state pension. A full state pension is worth £8.5k p/a (and it goes up with inflation). To get that level of income in a private scheme you need to amass somewhere around 250k-£300k. We know that most retirees don't have a private pot anywhere near this big, yet that is the "golden handshake" the government is giving to each and every retiree. That level of unfunded obligations is why the state pension is surely destined for collapse in the future.