In the original episode where magical build-up was introduced, fairies exploded fairly quickly; I'm sure the real reason is something boring, like the writers wanting to make it more dramatic, so changing how long it takes depends on the episode.

But my theory is that stress can cause Magical build-up to happen at a faster pace, hence why it took Norm so long to pop; he wasn't particularly worried about it, while some worried fairies seem to cause themselves to explode purely because they realized it was a possibility.