How much did he see them before the split?
I am no expert and I am only going on the advice I have been given and have read, but the way I understand it is the less they are used to him as a caregiver, the more frequent but shorter the periods of time are advised to be.
My problem with My H wanting to do 3 nights is before we split, he worked 6/7 day weeks and would probably only be around at bedtime 1/2 nights a week. Even then I'd still do bath and story most of the time.
He did see him every day though and always spent a good quality hour with him at some point, but three days and nights is such a big leap and such an upheaval for DS IMO.
I am prepared to build up the time very slowly though, but TBH, although I was very worried about him maybe having to be away for so long, I don't actually think H will ever sort his work out enough to go through with it all.
And of course if when DS is at school and he wants to see his dad more I won't stop him.
The lady I spoke to was actually a relationship counsellor I went to for a session, but I knew she worked with children and spent a good 15 minutes talking with her about this.
She is on,y one opinion I guess. Im not sure if there is an official body who can also advise?
I found some interesting studies on the net, I will try and dig them out later and post links. It just helps to know what is best for children at different ages.