Ritual spell casting

Telecanter posted a PDF with some of his favorite house rules, which reminded me of one I've been considering. I'm not completely comfortable with the way fourth edition separates magic into combat spells and non-combat rituals, but I do think there's a place for ritual casting in OD&D. I like the idea of the PC's bursting into a room to interupt an evil priest muttering over a smoldering brazier and a bound sacrifice. I'm not sure exactly how it should work as a rule, but I'm starting with this on a trial basis:

A magic-user (or cleric) can cast any spell they know without it occupying a daily spell slot so long as they have the spell in written form and can take the time to cast it (two 10 minute turns per spell level). This is not something they can do in combat or any other distracting situation. The referee should make a wandering monster check each turn during ritual spell casting.