What ATPG said.

One other thing that's important in the conversion decision is that you don't want to start converting settlements that are well developed (both castles and cities), which means that generally you don't convert post turn 50 or 60 as then you will hardly regain the total loss in time for it to matter.
The best time to convert is within the first 20 turns when things are relatively undeveloped and frontlines still mobile.