Hi gatorgirl,
Normally 30 days or so would be a good amount of time to make a good assessment of a new mattress but of course if your experience makes this impossible then shorter than that may be necessary.
Do you have a mattress protector or a mattress pad or anything else on top of the mattress (besides your sheets of course)? This can also make a difference in your experience or how well the mattress contours to your body profile and/or relieves pressure.
I would also keep in mind that when you change a mattress then you may also need to change your pillow because this can also make a difference in upper body “symptoms” and shoulder pressure as well.
Post #2 here has more about some of the more common symptoms that people may experience on a mattress and some of the possible reasons behind them (although each person can be unique and experience similar symptoms of discomfort or pain for different reasons than someone else).
Phoenix