Oxon Megafinal uses clocks on all boards except lower U7s, or unless we run out. We are one of few, I think, that uses 30 mins each. This year I'd intended to introduce incremental controls for older sections after some inexperienced controlling last year led to controversy on 2 min rule.
I know of other Megafinals in adjacent counties that already use increments but shorter time controls.
In the schools I teach in I tend to use 20 mins each for qualifying rounds, but for some I don't use clocks at all for that stage.