I’m not sure if there’s a time limit you have to be within in order to get to Aslan’s Country in the end. But I think if there isn’t, then eventually Susan would have made it there. There are always different phases in life that everyone mostly goes through (with some exceptions, of course) including a rebellious phase. I’m not sure Susan was going through this type at the time that she decided that Narnia doesn’t exist, but it must have been something quite selfish to make her so adamant that a place that she actually visited and, in fact, ruled over wasn’t there. In any case, people tend to mature a bit as they age and so I do believe that over time Susan decided that she was being what C.S. Lewis called her and turned away from that path and toward accepting the fact that everything she experienced was real.
To know you are ignorant is the beginning of wisdom.