I have a "20" minute rule in my department. If is taking longer than 20 - 30 minutes to identify the issue and come up with an initial solution. Get another set of eyes. Inevitably it is resolved quickly or it is determined to be a major issue and we can make adjustments (like have the user move to another machine or switch to backups, etc.). Nothing worse than having someone spend ALL DAY on something that can be fixed in 10 minutes.