Complex sentence: Pressing a button on the watch would make him invisible again which was a helpful tool in his line of work, but he could only do this for 15 minutes per day or else he would become permanently invisible.
Split into simpler sentences: Pressing a button on the watch would make him invisible, which was a helpful tool in his line of work. He could however, only do this for 15 minutes per day or else he would become permanently invisible.