I know there is variation in the civilian world, but I think you should assume that the military will make you do a 3-year cardiology fellowship followed by a 1-year interventional cardiology fellowship.
Fellowship payback is 6 months for 6 months with a minimum of two years. The rub is that the military runs cardiology fellowships, while I don't think it operates interventional fellowships. I think that means these would be considered separate entities, so 3 years for the cardiology fellowship and then 2 years for the distinct 1-year interventional fellowship.
With IM residency, that brings your GME obligation to 7 years, which is paid back concurrently with an HPSP or USUHS commitment. This also assumes continuous training.