I composed a response earlier which I do not see now. Boiled down to Your Mileage May Vary. One that had built up a full head of steam might get significantly larger in 5 years. One statement I have seen was to the effect that fully fueled bamboo will triple the size of its parts every year until maximum size (for that site) is reached. Likewise the rate of spread should increase as the plant gains in strength. However, runners can pop up unexpectedly some distance from original plant without any shoots between.