Mana is measured on a scale of 1 to 10+  When Ilyana was transformed her mana had to be awakened as part of the process.  Ilyana started out in the high '1' range for her mana level.  To raise it to a 2.5 in just two months is unusual and means she has an affinity for it.  Being able to cast spells is the ONLY thing that Ilyana likes about what was done to her when she was brought to Trivium.