  Estimation of multiple parameters in an unknown Hamiltonian is investigated.
We present upper and lower bounds on the time required to complete the
estimation within a prescribed tolerance $\delta$. The lower bound is given on
the basis of the Cram\'er-Rao inequality, where the quantum Fisher information
is bounded by the squared evolution time. The upper bound is obtained by an
explicit construction of estimation procedures. By comparing the cases with
different numbers of Hamiltonian channels, we also find that the few-channel
procedure with adaptive feedback and the many-channel procedure with
entanglement are equivalent in that they require the same amount of time
resource up to a constant factor.
