Before revision:  We demonstrate that timescales for the nucleation (resp. closure) of an approximately 10 base-pairs bubble, in agreement with experiments, are associated to the crossing of a free-energy barrier of 22~k_{\rm B}T (resp. 13~k_{\rm B}T) at room temperature T.
After revision:  We demonstrate that timescales for the nucleation (resp. closure) of an approximately 10 base-pair bubble, in agreement with experiments, are associated to the crossing of a free-energy barrier of 22~k_{\rm B}T (resp. 13~k_{\rm B}T) at room temperature T.