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.