Hi New2Latex,
It can vary depending on the person and on the specifics of the mattress and the cover but all new mattresses will have a break in and adjustment period of a few weeks although for some people it can be much shorter and for others is can be much longer (see post #2 here and post #3 here). At a bare minimum I would give any new mattress at least two weeks (and hopefully longer) before deciding whether you need to make any changes.
I would tend to agree with Shawn based on averages (see post #2 here) and he is right that most primary stomach sleepers will generally do better on a firmer mattress to reduce the alignment risk of sleeping in a swayback position (see post #4 here). Many stomach sleepers will also do better with a thin pillow under their pelvis/lower abdomen) to prevent their pelvis from sagging into the mattress and hyperextending the lumbar spine but each person is unique both in terms of their physiology and the specifics of their sleeping positions so your own personal experience is always what really matters and will always “trump” theory or the “averages” of others that may be similar to you and do better on a different combination of layers…
Phoenix