Interesting. The book differentates burning from consuming fuel from heat. (Cause, well, it can?)
I'm not sure I see the rego, since you're changing the fire. (Which, itself, suggests muto). One change I could see would be mutoing the fire to consume a /different/ fuel source while burning on the primary. The problem is that fueling a fire is, in some way, perfecting it? What aspects of creo would there be to "maintain a fire?" What parallels can we draw between other forms?
One of the properties of a fire is to consume fuel similary to its properties to produce heat and light , any of these properties can be changed , controlled or removed with magic . Muto , Rego or Perdo can all be used for this effect. Creo could produce a magical flame which does not use fuel but in the situation described it would not be useful.
A CrAn(He) Spell could be used to 'heal' the candle however it would be a ritual to create a permenant effect and need vis so a simple low level MuIg is probably hte easiest
Candle which burns for a day
MuIg 15 (Base 4 +1 range touch. +2 duration sun)
Removes the need for a candle to use fuel for sun duration
Candle with no fuel Pe Ig 25
(Bse 10 +1 touch , +2 sun)