Not sure why everyone is talking about you moving as your question was how long after her 'moving out' can she still decide that things are hers.
Surely at the point when they divorced it was agreed who had what as far as their joint property is concerned, so unless DH agreed to store the stuff for her, it is his now not theirs (as in their 'joint property'). If this is the case she can only have it if neither of you want it and are prepared to give it to her in the same way as you might to any pregnant friend. She has no more right to it than any other 3rd party just because it was hers "when they were married". Of course, if he was storing it she should now arrange for it ALL to be collected in one go.
I had experience of this when I moved in with my now DH, three years after he was divorced. Even after I was here his ex was still using every chance she could to get into the house when his 19 yr old daughter was here (and we were out) to "collect things I forgot to take". OK they were only stupid things like a bun tin, or a carving fork, but it was really frustrating to be searching for something and DD to say oh Mum took it as it was hers. Fortunately I saw the funny side of it but DH was really mad about her disrespect and words were said....
But what if your DH's ex suddenly decided she wanted something else from your home other than baby things? Would you even be in this dilemma. Probably not because it would be easy to say no.
In this case, as the stuff from the previous marriage that she didn't take is now the property of DH, it is up to him to decide if he wants to make the effort to go into the attic to at least look at it and decide if it is feasible for you to pass it on to her next week when you see her. If not, it is up to him to say to her why not.
The other thing I don't quite agree with are the comments from people asking why you would want to use the same baby things she did. What I really don't understand is why she would want to. She is now not with DH or the child who used those things, why would she want to stir up all the unhappiness from that time in her life.
I agree with the others though that having a child in the house is not a reason or excuse for avoiding this. You don't both need to go up there, so you look after DS and DH goes to look. If there are things he wants to pass on, you and DS can make a game of receiving the stuff at the bottom of the ladder.