How long can bentos be left out?

I'm new to bento creating, but I've kept up with Pikko's Adventures in Bentomaking blog for ages. That's what originally peaked my interest as well as a desire to moderate my portions. I feel like I'll have no problem with the cooking and creating aspects of bento boxes, but I have one issue that I haven't been able to find answered anywhere: How long can bento boxes be left out? Fridge and microwave access at my work place aren't a problem, and I know that there are quite a few options for microwaveable bentos. But cold rice just isn't that fun (or yummy), and it'd be a shame to create a hot lunch only to take it out of the fridge 3-4 hours later. This is assuming that you have a basic bento creation of rice, a side of meat and cooked veggies, etc. So, can a bento be left out for a few hours, or should I just invest in an insulated box/bag/thermos type bento?