  Ultralight vectors can extract energy and angular momentum from a Kerr black
hole (BH) due to superradiant instability, resulting in the formation of a
BH-condensate system. In this work, we carefully investigate the evolution of
this system numerically with multiple superradiant modes. Simple formulas are
obtained to estimate important timescales, maximum masses of different modes,
as well as the BH mass and spin at various times. Due to the coexistence of
modes with small frequency differences, the BH-condensate system emits
gravitational waves with a unique beat signature, which could be directly
observed by current and projected interferometers. Besides, the current BH
spin-mass data from the binary BH merger events already excludes the vector
mass in the range $5\times 10^{-15}\ \mathrm{eV} <\mu< 9\times 10^{-12}\
\mathrm{eV}$.
