Two low level (1-6) spells can be cast per melee.This is a good basis, but there's got to be exceptions. In the first statement, on low level spells, it works out that one spell can be cast per two or three melee actions. Simple spells from levels one and two should only require a single melee action to cast. Most combat spells should also require one melee action to cast - spells like Time Slip, Call Lightning, Armor of Ithan, Fire Ball, Magic Net, Multiple Image, etc. Some spells might have a stated casting time in the description, and that should be used instead.
One mid level (7-10) spell can be cast per melee.
One high level (11-15) spell can be cast per two melees.
Some incantations must be performed as a ritual.
A simple (level 1-2) ritual will only require 1d4 minutes to perform.Runes, mystic symbols, wards, and all symbol- based magic must be created before use and should take 3d4 minutes per simple symbol and 6d4 minutes for complex ones. Activation of such symbols is almost always quick and simple - a single melee action is needed to do so.
A low level (3-6) ritual will only require 3d6+2 minutes to perform.
A mid level (7-10) ritual will require 5d4+10 minutes to perform.
A high level (11-15) ritual will require 1d6x10+15 minutes to perform.
Some rituals might have a stated time in the description - if so, go by that.