Quilgo is an excellent tool that allows you to time quizzes created on Google Forms. You can specify the duration of the quiz and choose whether or not you want to force forms to close when the time is over. Keep in mind that if you choose to have forms automatically close after the time is over students will have to submit their forms within the allotted timeframe or else they can not. When you do not activate this option you will be able to see how long students spend overtime working on their quiz.
Quilgo also offers camera tracking (premium feature) allowing you to see video snapshots of your students while they are working on their quiz. With this feature on, you will be provided with details on where students channeled their focus and whether they were distracted or not as they were working on the quiz. Once students submit their forms you will get access to detailed data about their submission with the ability to export data for further analysis.
Sons of Technology has done a wonderful job explaining how to create timed exams using Quilgo. Watch the video to learn more.