For the sake of concreteness and specificity, let's say that both parents work full-time Monday thru Friday.
What is reasonable for the remote parent to expect from the 'on site' parent?
Some criteria that I think is reasonable:
I think  is probably the least reasonable (but still reasonable). In my mind, children, especially young children, should have a fairly fixed or 'structured' schedule, particularly on school nights, so there should be several 'natural' opportunities for this to be scheduled, e.g. after eating.
1 Yes, things come up, things happen, and sometimes it might be infeasible, or even impossible – that's fine. But five (5) minutes is very minimal and could (or should be able to) be squeezed in easily any night, e.g. before the on-site parent starts putting the child to bed.
2 Any longer amount of time – within reason! – is great tho.
While I agree with the OP that the proposed schedule looks reasonable, there is one more factor, esp, if the two parents involved are not on amicable terms.
Six year olds tend to be quite distracted, and may not be in the frame and actually respond on a device, even if its for 15 mins. In our house, this actually ends up being additional work for the parent because that parent has to follow the kid with the device in hand, trying to get responses from the kid.
Other than that, this seems like a interpersonal question which you'd have to resolve with the other parent.
From experience, I recommend:
Avoid video chat shortly before bedtime, just as with any other screen time, to promote better sleep. Maybe also avoid phone calls shortly before bedtime, especially if you notice that this makes the child more excited and less easy to fall asleep.
Talk every day or every other day.
The child's attention span, as well the the time constraints of the parents, are very individual, and vary across days. We had success with video chat and phone calls that last between a few minutes and half an hour per day.
Meal time (such as dinner) that is not under time pressure is a good choice for a chat. It is usually sufficiently far from bed time. Also the phone or tablet can be positioned on the table, and, at least for some kids, this makes it easier to "follow" them with the device (they are right in front of it, eating and talking). Family dinner plus in-person conversation is a great tradition, and dinner plus a video chat or phone call a good enough substitute, given that one parent is away.
Avoid making the other parent follow the child too much with the phone or tablet, because this may get annoying for them.