I've got a microwave container and no oven and looking to cook some stuff - is it possible to microwave a container that's marked as microwave safe for 40 minutes or so?
A microwave-safe container shouldn't be heated significantly by the microwave (only by conduction from the food it contains). So, it should be fine, so long as the food is, and as long as the food doesn't exceed its allowable temperature.
You can test fairly easily—put the container, partially filled with some water, in the microwave for a few minutes. Does the container heat up, possibly more so than the water—especially where its not in contact with the water? If so, I'd be reluctant to use it in the microwave for that long.
Some containers are microwave safe, but only for heating to maybe 160°F or so. A common example is LDPE (often used for plastic wrap). Some can't take boiling for more than a short time (e.g., HDPE). Polypropylene should be fine, even with boiling. Go any hotter (e.g., filled with oil, which is probably insane) and most will fail. If you're lucky, the container will have a resin ID code on it, LDPE is #4, HDPE is #2, polypropylene is #5.
Your microwave may not be fine with 40 minutes of strait microwaving, or with the steam buildup. You should probably check its manual.
I assume also you've read Jay's comment about converting recipes, and that you're doing something where 40 minutes won't result in charcoal.