Hi soretom,
When you first purchase a mattress I would try to sleep on it for a month or so (or a minimum of a couple of weeks if longer isn’t possible unless it’s obvious that the mattress or your pillow is completely unsuitable for you) so that the mattress has a chance to break in and your body has the chance to adjust to a new sleeping surface that is different from what it is used to (see post #3 here).
After the initial few weeks if you are making “fine tuning” changes to your mattress or your pillow I would sleep on each new configuration for a few days at least (or longer if there are clear signs that things are improving) so that you have a chance to identify any clear patterns in your experience because your experience for just a single night or two can often be an anomaly or the result of your body adjusting to the change itself.
Phoenix