You could store the time when the user opens the form and store it again when the user saves the form and show the difference.
Here's how you can do it:
1) Create a hidden field called startTime and put this JavaScript in the docReady event of the field:
this.rawValue=(new Date()).getTime();
2) Create another hidden field called endTime and put the following JavaScript in the preSave event of the field:
this.rawValue=(new Date()).getTime();
var dif = this.rawValue-startTime.rawValue;
var d = new Date(dif);
totalTime.rawValue="Hours: "+d.getUTCHours()+" Minutes:"+d.getMinutes()+" Seconds:"+d.getSeconds();
3) Create a third field called totalTime.
Kyle