At my old job, we built, maintained, and operated a huge customer-facing system. We had 2-3 big releases per year, and in every release cycle we devoted around 20% of our time to maintenance, whatever time was necessary for critical bugs (generally from the new code released in the last cycle) and the rest to new development. We (the dev team) put this split into our estimates and development plans, and we had official approval from the project management team and c-level execs. That 20% time was ours to use as we saw fit, and we generally used it for refactoring code we felt was hampering productivity on new development. But we also used it for side projects; I built a performance/metrics gathering and reporting system that was incredibly useful to me, and ultimately became incorporated in marketing materials and the ceo's presentations.

We were lucky to be able to use this 20% time openly, but if we weren't we would have increased our new dev estimates to get the time.