Hi SuperJ,
Outside of any changes over the first few days when a mattress is decompressing … there will also be a break in and adjustment period for any new mattress or sleeping system as the mattress loses any of it’s “false firmness” and the cover stretches and loosens a little and the materials settle and your body gets used to a sleeping surface that is different from what it is used to (see post #3 here). This would typically be a few weeks but it can be shorter or longer depending on the specifics of the person and the mattress.
It can be quite surprising to some people how much their sleeping experience can change over the first few weeks of sleeping on a new mattress (or mattress/topper combination).
As you know Ikea has a 90 day exchange policy so I would give a new mattress a few weeks before exchanging it for something else (or longer if your sleeping experience is still improving) but I would certainly make sure you exchange it before the 90 days are over if your “symptoms” are clearly getting worse or if you are getting closer to the deadline and you still aren’t sleeping well.
Phoenix