I have left a pen out on the counter for upwards of 12 hours on several occasions with seemingly no ill effects. It would be good if your pen is more than half used, that way if it did go bad not so much would be wasted. Remember these pens are meant to be used at room temperature, so it's not going to hurt it immediately. The problem is that we use such small amounts that pens need to last far longer than they do for people.
I bet it would be fine for that long though, as long as it's "room temp" and not 85 or 90. If you can, get a small cooler and put some ice packs in it, but wrap the pen in some bubble wrap to keep it from direct contact with the ice. Freezing is more harmful than being at room temp, which would be 70s.
