People who consistently invest 15% of income are able to retire in 60s and maintain their standard of living. That 15% can be invested in any retirement account (401k, IRA, 403b, etc). The 401k is the most common, allows you to invest more than IRA each year, and is funded through payroll deduction so you don’t have to remember to transfer funds, so most people recommend funding the 401k first.

If you invest less than 15% for a couple of years, you’ll likely need to invest a little more than 15% later. Ideally, you stick with 15%, but some people need to adjust percentage to save downpayment in a reasonable time period.