20×10%
My previous employer used to plan "express checkout" weeks for developers. In between two larger projects, you'd get the chance to spend two weeks doing nothing other than picking low-hanging fruits off the bug-tracker tree.<p>It worked miracles, with minor-but-irritating bugs getting solved much faster, and with features for niche groups getting implemented as opposed to laying in wait for a bigger project it would thematically fit in.<p>Some devs even went out of their way to spend those two weeks on a single, irritating, deeply-rooted bug that nobody in their right mind would even want to fix.