If I make a batch of iced latte, that is to say I make a regular latte then pour it over ice, how long can I keep a batch fresh in the fridge? A day? A week? If the milk is the limiting factor, could I make a batch of espresso, chill that, and add milk as and when to get the same drink, or would it taste different if prepared this way?
When I worked in a coffee shop and we prepared a beverage similar to an iced latte, we always mixed the milk in with the chilled espresso. Keep in mind this was a beverage prepared ahead of time and not server to the client right away.
We NEVER added dairy to espresso and then chilled it for later. It has been shown that 1 liter of milk can develop harmful bacteria after being in a room temperature space for as little as half an hour.
I would highly recommend that you do not prepare your iced lattes completely with milk and then chill them.
I recommend making the espresso, adding whatever sugar or flavor you want while it is still hot, chilling it in the fridge, and then adding milk to the espresso once it has been chilled or when you are looking to drink it. This will keep for a maximum of 48 hours.
As with all coffee, the fresher the better, but if you are indeed looking for a quick way to grab an iced latte, I would do so in the manner I described.
I mix instant expresso granules, with milk and sugar, and refrigerate. It usually only last a day, because i drink it all up, but i made a double batch last week, and had 2 litres in fridge that kept no problem for 4 days, tasted fine, needed shaking to remix it up after sitting, but it was great. I make enough friday to last the weekend. I mix 2 teaspoon expresso with a small amout of milk (about 100ml), to make a concentrate, add 2 teaspoons of sugar, microwave for 20 seconds (Enough to dissolve sugar), then mix in 1 mug of cold milk (about 400ml), and pour that into an empty milk container and repeat, the last one i make that fills the container to the top, i don't add the sugar, then shake the whole thing up and bung it in the fridge, and enjoy at your leisure. If its not strong enough, i add more expresso granules.