We consider a non trivial Boolean model $Σ$ on ${\mathbb R}^d$ for $d\geq 2$. For every $x,y \in {\mathbb R}^d$ we define $T(x,y)$ as the minimum time needed to travel from $x$ to $y$ by a traveler that walks at speed $1$ outside $Σ$ and at infinite speed inside $Σ$. By a standard application of Kingman sub-additive theorem, one easily shows that $T(0,x)$ behaves like $μ\|x\|$ when $\|x\|$ goes to infinity, where $μ$ is a constant named the time constant in classical first passage percolation. In this paper we investigate the positivity of $μ$. More precisely, under an almost optimal moment assumption on the radii of the balls of the Boolean model, we prove that $μ\textgreater{}0$ if and only if the intensity $λ$ of the Boolean model satisfies $λ\textless{} \widehatλ\_c$, where $ \widehatλ\_c$ is one of the classical critical parameters defined in continuum percolation.