Assumptions make projections for important financial indicators into an uncertain future.
In fact, the SSA estimates that 25% of them will live to be over 90.
That means you jointly have to come up with a good number for this.
Returns on fixed income securities like US Treasuries and corporate bonds are known to investors and published on financial websites. A mixed portfolio of stocks is historically expected to return 10% per year, but that is a long term average that includes wild upswings and devastating crashes.
It is best to project current returns into the future. The stocks traded on major exchanges number in the thousands. Financial publications and websites can offer guidance on the direction that stocks will take in the future and the returns that can be expected.