An alchemist that gets robbed of one or more extracts should probably only need to wait 1 day to recover the previously occupied extract slots
On alchemy says, "An extract, once created, remains potent for 1 day before becoming inert, so an alchemist must re-prepare his extracts every day." This GM would rule that an inert extract isn't really properly an extract anymore but just a useless concoction of once-magical ingredients. So, in light of the discovery infusion, because the extract no longer exists as an extract after 1 day, this GM would rule that after 1 day the stolen extract no longer occupies one of the alchemist's daily extract slots.
However, were another GM to rule extracts remain extracts beyond 1 day, the alchemist could see that extract slot occupied forevermore if, for instance, the extract thief, while exploring alone, fell into a pit and died, stolen extract in his belt pouch. That would be stupid and terrible, especially considering the tendency of some GMs to have the PCs robbed or captured and stripped of their gear. If the GM rules this way and possesses such tendencies, this player recommends picking a class other than alchemist or, at least, a discovery other than infusion.