Frequency varies based on water quality and usage.
In general Boilers should be blown down (drained under pressure) daily. Then at least every 3 months the boiler tank and all orifices should be cleaned to remove lime buildup.
2 years will be a lot of lime and will be hard work to hammer and chisel the lime to be able to remove the heating elements. You can use a delime product to help with fittings and limed parts (check out Scalebuster
One of the best investments you can make to help eradicate lime is installing a water conditioning system (check it out Water Treatment System