How long will 100 watt bulb last?

Read the label on the container. The light bulb will be rated "B-10" life. This refers to a statistical number value. "B-10" Life is the average life span a bulb will last for 90% of those tested. If you went purely by the "average" time to burn out, then that is not specific. One brand of light bulbs can have a higher Average. But if it has a lot of scatter in the data, then that would mean some bulbs could fail much earlier and some much later. On the other hand, if you determine a bulb has a X hour "average" life with a minium scatter factor, then more of the bulbs will perform closer to the average. Read the label on the box.