The average duration of the pow is determined by the minWeightMagnitude (i.e the difficulty of the puzzle to solve) and the computing power of the device.
On Bitcoin, the difficulty of the pow is adjusted is such a way that there is a new block every 10 minutes (on average). (basically it is driven by the total computing power of the network)
On the Tangle, the difficulty of the pow (minWeightMagnitude) should be adjusted in such a way that "problematic spam" became so costly that it is abandoned (or at least don't arm the tangle significantly). Basically it is driven by "adoption rate" :
To simplify, the tangle have (at least) 2 ways to protect against "problematic spam" : the minWeightMagnitude (i.e. the computing power required to attach to the tangle) and the mass adoption (a tangle with many valid transactions is more difficult to arm)
If we imagine that IOTA became a money that can be used in the supermarket, it means "huge adoption", it means "huge amount of valid transaction", it means that spammers need to produce more and more invalid transactions to significantly arm the network. It means that the minWeightMagnitude can be reduced.
Regarding usage in the supermarket, remember that the first target of IOTA is micropayment in the machine to machine economy.