The insert that came with the hospital grade medela pump I rented right after birth said up to 8 hours at room temperature, but I never left it longer than 6 (and usually less than that). If it's really hot though I would say for a shorter period of time. If it's in a cooler bag with an ice pack thing it should be fine for the day.
On a side note though, has your lo taken a bottle before? Not all babies will take one easily. Also, with your lo being so young there's the chance of nipple confusion, though I don't think this is that common. We had to give some bottles of formula top up the first few weeks before my supply was up and never had any nipple confusion (though she did develop a lazy latch we had to correct).