There are no guaranteed returns on anything, even US treasuries have returns that vary over the years and decades, and that assumes Congress never pushes the US into default by refusing to increase the debt limit...

In my personal projections, I assume average nominal annual returns of 7.5%, which is about 4%-5% real return. This despite having an average nominal annual return of 10.4% over ~20 years, simply because I'd much rather underestimate rather than overestimate.

Having said all that, I'm starting to invest in a specific PE investment that targets 15% return net of fees and that has a track record of +25% returns for the past 10 years or so.