If you are on a conceptual or basic engineering stage, this is one of the main typical questions. As far I know, the time between decoking is one of the key differences in vendor technologies. It is a function of your feedstoke, of course. Can be 3, 4, 7 months.
Operational issues were excellent described by the other member in their answer: TMT, coil pressure drop, coil material metallurgy, etc. AND you operate the heaters: are you operators well trained on combustion?
There are other operational methods to decoke, like pigging, BFW, Steam-air decoking, on-line spalling, etc.
depending on experience, the vr properties also plays a role in determining the heater run lenght together with good operation. We had seen cokers with somewhere 2 months to 1 year before going into spall.
We have run our Delayed Coker unit for 543 & 531 days respectively for two consecutive terms without online spalling or steam air decoking of our heater.The run length of the heater depends on many things e.g. feed stock quality, severity of process, quality of firing in the heater & number of unit interruptions taken place. The run length can be maximized by close monitoring of process parameters, good heater operation with uniform & proper firing of burners and prompt tackling of emergencies which may result in heater / unit interruption.