First hours.
A mineral oil has a very low film strength which allows all moving parts to wear out faster for a better fit. Imagine the cylinder of a new engine to be nicely round and the piston ring a bit oval. The idea behind breaking the engine in is to have the piston ring take the shape of the cylinder which results in less friction. Less friction = less heat = higher performance. Because most synthetic oils are actually much better lubricants than mineral oils, this process will take much longer and therefore the chance of damaging the engine during the first runs is higher.