How long does it take to break in this memory foam mattress?

Give it two to four weeks. A deep memory-foam bed takes time for your body to adapt to its slow-conforming feel, especially coming from a firmer or spring mattress. Because the feel settles in over those first weeks, the 365-night sleep trial gives you a full year, across every season, to be certain before committing, far longer than the short windows many sellers offer.