[Steve Plambeck]

Much in the spirit of what Colin proposes, but from just a slightly different approach, I would handle it by doubling (or tripling, etc.) the amount of time it would require for the fatigue spent on spell casting to wear off.

From the practical standpoint, it means none of the spell costs ever change, and there's no fractions or rounding to ever deal with for spells with odd number costs or continuing costs.

From the in-world cosmology standpoint I favor this because I treat spell costs as paid for in Mana rather than ST, so if the ambient Mana needed to be accumulated to get the wizard "recharged" is thinner, it should take longer. (Resolving this with pure TFT rules simply means equating the ST spent with the energy required to draw on or suck up the Mana.)

Spell fatigue taking longer to recover may be considered analogous to physical fatigue taking longer to recover if the party was traveling through a mountainous, high altitude environment with a very thin atmosphere.

(All credit for this goes to Larry Niven and his novel "The Magic Goes Away".)