People are usually talking about baby's and very young children (toddler age) when they talk about building up, OP. Cases in which the father has hardly had the child at all, because it's been breastfeeding or something so he hasn't been able to have overnights.
Unlikely to need to build up when the child/ren have an established relationship with their father, and when he already does overnights regularly. Months building up from this would probably just be more stressful for the children, because it means more changes happening over a longer time than just adjusting now, and means that no set pattern can be figured out because it keeps being added to.
If the case is definitely going to court (as in ot is not just him blowing hot air) he will almost certainly get 50 50 if that is what he is asking for, and if there are no serious concerns for the children's wellbeing. As he already has them regularly, and overnight, that is unlikely to be the case. So it would be best if you figure out a proposal now, rather than hoping he doesn't get 50 50 and worrying about it when it happens. He almost certainly will. As he should, as his children's equal parent.
The age of the children might be important for determining a set schedule, so if they are 3 and 8 now it night need to change every few years to accommodate their needs at that stage. Week on week off might be too much for a 5 YO for example, but work well for an older child, so you could do alternate weekends and then split the in-between days.
For example Monday afternoon to Wednesday morning with mum, Wednesday afternoon to Friday morning with Dad, Friday afternoon to Monday morning with mum. Then the second week is the same schedule but with the opposite parent.
You will need to decide on things like whether belongings will move between the homes, and if they do which belongings? For example how will school uniform work, with washing and moving between homes, if the handover is just done by the parent whose day it is getting the child from school, rather than parent to parent.
You will need to be able to communicate about things such as medical appointments. For example if mum makes a dentist appointment, but can only schedule it for the dad's days.
If the child is ill, do they stay at the parents house they are at until better, or do you move them whilst poorly?
How will special occasionss be handled? For example the child's bday, or mother's/father's day. Will the schedule stay the same, or will it be flexible/alternate?
There is a lot that will need to be decided. So of this is definitely going to court it would be best for you to decide how you envisage this working, so you can immediately present it as an option when he is awarded 50 50.
If you really feel it is important to build up to 50 50 then the best you can really do is attend mediation, or work it out privately before it goes to court. Accepting that he will get 50 50 regardless, and at least this way you can save yourself the court proceedings and the abrupt start to the new schedule.