i believe as often as posible but also think that just turning up when ever is not good. they need to get in a routine. like stay with mommy monday to friday and go with daddy saturday morning till sunday evening, or if you cannot have them over night for any reason then you have them every saturday or sunday. this way the children will no what day they see you .
my partner has his son every other weekend because the childs mom works full time so this way mom gets to spend a weekend with the child too.
Both parents should reach a decision on this one and both should stick to it.
However much the father wants to see the children i think, however it should not interfere with schooling etc.
Preferably as often as the children want to see him, if that is possible. And if he can't see them for reasons of distance, etc. the children should be able to have free access to him by telephone whenever they want to speak to him.