The general consensus is that it shouldn't be about what is 'fair' for the parents, it's about what works best for the children.
Mine stay with their dad once a week, on whichever day works with his shifts. I'd prefer a more regular day and the DCs would prefer to have 2 nights with him, but due to his work this is how we do it.
DP has 50/50 with his ex, so a set 3 nights each and the other night alternates each week.
It depends on your living arrangements, work commitments and whether school is a factor (does ex live close enough to do midweek school runs?) as well as what makes the DCs happy.
There's no point doing 50/50 if one of you works until 8pm and they don't get to see that parent on 'their' night. In my opinion, they would be better with the other parent if they are at home.
However, each family is different. What matters is that your children are as happy with the arrangement as they can be, are they able to have a say in it at all? Do you and the ex get on well enough to all sit down together and discuss it?