Variant Rule - Spell Recharge

I am tearing my hair off my head to make an Unearthed Arcana-like variant rule for this game: spell recharge.
the basis is to assign a recharge time to spell accordingly to spell slots, time that is determined in a semi-random way (min and max value from which choose, which I presume is calculated when necessary).
Anybody has an idea how to make it?

EDIT: as a clarification, http://www.d20srd.org/srd/variant/magic/rechargeMagic.htm