During the press conference [1], they stated that there was a variety of models that depended on the physical properties of Dimorphos, which were not well known.
The estimated range by which the orbital period would be shortened was between "a few minutes and several tens of minutes" and that 32 minutes is "consistent with the estimates but at the upper end of that range".
They also showed a pretty fresh Hubble image of debris plumes from this past Saturday and mentioned that teams are working to use the data and imagery to better understand the composition of Dimorphos.
[1] Discussion starts around 11:25 in this video: https://www.youtube.com/watch?v=Zhzn0U2m5wQ&t=685s