Is it acceptable to only provide pressure relief for ulcer when they’re tired?

As willie said, no this is not exveptable. Bed sores are very serious. I would talk to the Director of Nursing (DON) and tell her/him that you want a woundcare nurse to look at Mom. That you have requested cushions and that she not be in a wheelchair all day. This is not unreasonable. If you buy a ROHO cushion make sure the persons name is on it or it will disappear and they r not cheap.

You may have to purchase this pressure cushion yourself. They exist and are VERY expensive so you will want to mark this with an indelible pen.
Laying in bed will create pressure ulcers as well, and put her in danger of pneumonia.
Pressure ulcers are deadly (sepsis) and are taken very seriously for any institution and they can lose licensure from having too many. (They are ALSO almost inevitable with aging issues).
Report to the ombudsman today. You can also report to licensure board or tell your facility you intend to do so.
MD should now order a wound care team consult; if he hasn't done so that is basically negligent.

NO THIS IS NOT ACCEPTABLE. When they noticed a stage one pressure sore on my mom's tailbone she was kept in bed as much as possible for at least a week.
I also bought my mom a ROHO seat cushion for her wheelchair, it was very expensive but since she spent all of her awake time in that chair it was a necessary medical expense.

I bought a sleepavo brand cushion for my mom. Her tailbone / sacral spine was also very sore pretty much forever. The sleepavo was super comfy, helped her guide her legs to a good position while sitting and her tailbone wasn't being touched by anything. (theyre on Amazon).

Keep pushing for action.
Bedsores are an awful problem & need appropriate treatment including offloading pressure.

Is there an OT on staff? OT can prescribe the right type of support cushion. Care plan can be ammedend for regular lay downs periods.

PS if Mom refuses the lie down each time, staff may not be able to insist upom it