Fragmentation timescales for the SN-model; the dashed line marks the minimum number of SNe for a blow-out superbubble and the corresponding time until full fragmentation of the shell at the top of the bubble (for details see text and Tables 2 and 3). Left: double exponential layer with H = 500  pc and n₀ = 0.5  cm^-3 (symmetric model); right: exponentially stratified medium with H = 100  pc and n₀ = 10  cm^-3 (off-plane model).