Constant REFRESH_FRAMES 

const REFRESH_FRAMES: usize = 256;

Frames a capped constant quality’s refresh of its coarse blocks (BlockLedger::refresh) would take at its first share of the region’s blocks a frame, which halves while a refreshing frame comes out past the peak’s frame budget and doubles while one comes out under half of it (RefreshPace): on a fast path the share doubles to the whole region in eight frames, and on a slow one the first frame is a small one.