Well you can create a very rough rule of thumb for yourself. Over a series of dives take accurate reading of the O2 you use and from this you can calculate the average O2 you use, (don't worry about accent etc. as this adds to the conservatism). Typically you create 0.9 CO2 for every Ltr of O2 you consume. This will based on your O2 used provide you with a CO2 production rate.
For CE certified Rebreather the duration of the scrubber is calculated on a CO2 production rate of 1.6 ltr minute (this is high). From this you can create a linear prediction on how the scrubber will last with your CO2 production (probably about 1lt
r/m). The fact the CE testing is conducted at 4oc again provide a conservative approach if you are diving in warm water.
I have found this calculation handy to determine scrubber requirements