  The valley degree of freedom in honeycomb crystals such as 2D semiconductors,
graphene and bilayer graphene is a promising carrier of quantum information
alongside spin and charge. This is all the more true since gate-controlled
single-particle quantum dots (QDs) have been demonstrated in bilayer graphene
(BLG), paving the way for the realisation of spin and valley qubits. Although
long spin relaxation times have recently been reported in BLG QDs, nothing is
known about single-particle valley lifetimes. Here we report single-particle
valley relaxation times ($T_1$ times) exceeding several microseconds in
electrostatically defined BLG QDs. The observed dependence of $T_1$ on the
perpendicular magnetic field can be understood qualitatively and quantitatively
by a model in which $T_1$ is limited by electron-phonon coupling. We identify
coupling to acoustic phonons via the bond length change and via the deformation
potential as the limiting mechanisms.
