If you've taken it out the fridge yes I'd use within 4 hours max, if it's kept in like a cooler bag with ice packs you can use within 24 hours. I only tend to use expressed bottles at home to avoid the faff of taking it out with me and keeping it cool!
After being in the fridge, 2 hours. 4 hours fresh 3 days in fridge 6minths in freezer
4 hours at room temp. So I guess just make sure ur bag ain’t near a radiator or anything.