the short answer: leave it in till it tastes the way you want it to.
The longer answer: when you are ageing in a barrel vs on chips there is one major difference. You will be getting a lot more o2 ingress in a small (I'm assuming 20 liter barrel?) barrel than in a fermenter + oak chips. This means that the ageing related changes to the beer that we want (softening of the hops, mellowing of higher alcohols into esters) AND the ageing related changes that we don't want (Papery flavors, muted flavors) will happen faster and to a greater degree. You can coat the outside of the barrel with wax to help reduce the o2 ingress.
However the advice to only leave it on the oak for 5 days is flawed. After about 5-10 days either way you will notice a pretty strong oak/woody/boozy flavor and will be tempted to take it out so that the flavor doesn't get any stronger. If you wait another couple weeks though the flavors will mellow and blend and the harsh woody oak character will drop out leaving the soft vanilla/whisky notes you are after with oak ageing. The woody flavor and tannins will bind with other compounds in your beer and drop out over time leaving the really nice wood flavors we want.