What is a good time to put my mum to bed?

A part of the aging process is sleep pattern disturbance, and it is worse with any setting in of dementias. I would say that I would in so far as possible allow her to follow her own sleep patterns. If she lives with you and is not in care this will be more difficult, and at some point placement may be a necessity so that the entire household isn't disrupted with sleep pattern issues. I sure wish you good luck, and hope you will let us know if you find something that works.

Because I work so early in the morning (5:30a) Mom and I have developed a routine that allows for us to both go to bed at the same time (around 9:00p).

She stays in bed for the most part (we have some middle of the night toileting) but she is usually really compliant. She even at times asks to go to bed early or will go put herself down for a nap. This seems to work for us.

Now waking up - that's another story - very difficult to get out of bed, even after a full night's sleep.

I guess the simple answer to your question would be to put her to bed when she's tired, but it sounds by your profile that she's perhaps getting on your last nerve by following you all around all day.
Does she stay in bed when she gets in it, or does she tend to wander? If that's the case then it may be time to be looking into placing your mum in the appropriate facility, where she will receive the care she needs and you can get back to just being her daughter and advocate.