  We build an optimal portfolio liquidation model for OTC markets, aiming at
minimizing the trading costs via the choice of the liquidation time. We work in
the Locally Linear Order Book framework of \cite{toth2011anomalous} to obtain
the market impact as a function of the traded volume. We find that the optimal
terminal time for a linear execution of a small order is proportional to the
square root of the ratio between the amount being bought or sold and the
average daily volume. Numerical experiments on real market data illustrate the
method on a portfolio of corporate bonds.
