when i did use those playtex insulated cups, i think they could go close to 3-4 hours!

but, they were a pain cause they had those little valves, so annoying to clean. i pretty much exclusively use those take & toss cups (dishwasher safe!

), and i think no more than an hour is okay (like the formula guidelines) - any more & i'd worry about bacterial growth & curdling.... ew!!
i have a couple small insulated bags (like a lunch bag) that i can put portable ice packs in for when i take milk on the go.