In a survey of around 30,000 renters by online rental marketplace Apartment List, around 79% of millennial renters dreamed about purchasing a home, but admitted that affordability remains a huge obstacle for them. That’s not surprising, since home ownership rates are perennially low among those under 35-years-old.

But how big of a challenge is it? Using data on starter home prices and the average saving rates for millennials currently, Apartment List found that renters in 12 major metropolitan areas would need a decade or more before they could afford even a 20% downpayment on a home.

That number is greatest in San Francisco, where homes are notoriously expensive. Most millennials would need to save for almost 28 years before they could afford a starter home’s downpayment, which could come out to around $142,800 in the Bay Area.

The West Coast, overall, is especially problematic for millennials, with Californian cities taking the top four worst spots to pay for a home in this survey.

The length of time needed for a home could also be a reason why younger Americans aren’t especially interested in saving for one. In Apartment List’s survey, 37% of millennial renters have not saved a single cent for a down payment, and more than 41% have not set aside a portion of their monthly income for a future home.

Last July, researchers at the St. Louis Federal Reserve recommended young adults postpone home ownership for years, if not decades. They asked millennials to “delay purchase of a home with its attendant debt burden until it was possible to buy a house that did not make the family’s balance sheet dangerously undiversified and highly leveraged.”