How long does formula last?

You have to keep it in the friday and no more than 24 hours.

the packet says use it as you make it but my midwife said to keep what he doesnt drink in that feed in the fridge and reheat to use again for the next feed but then it definately has to be tipped after that so we have been making up 200ml (we were buyin the sachets) and keeping it in the fridge and then tipping as much as we need into a different bottle and heating that as he wants feed. With all the feeds tho 200ml doesn't last long so havent had to worry about how long it should last

Yep stick in at the back of the fridge and then re heat it but after that youwould need to chuck what he doesn't drink.

I would make up smaller feeds - they can keep for 24hrs in fridge.

If he has already started on one bottle but does not finish the feed the left over formula should be thrown away - do not reheat and reuse as Redbedrock said there is bacteria in the saliva.

You are supposed to throw out after bubs has had some as bacteria can grow in it. If made up and un-used, it can keep in the fridge for up to 24 hours.
That said, I have given Gina re-heated formula when it's seemed like a shame to throw it out ie if I've made a full 260ml bottle and she's only had 10-20 ml. We've never had any probs but I don't do it as a general rule.