You can change the default meeting time to 15 minutes under settings. Once you have done that, just clicking defaults to a 15 minute meeting. You can already drag a 30 minute meeting to a 45 minute meeting pretty easily. This is close to what you want, but it does not solve the problem of being able to easily click and start a meeting at 9:15 with a 15 minute duration. Microsoft Outlook (desktop application) has a way to change the default intervals (example: 4 divisions show for an hour if you choose 15 minutes versus 2 divisions for 30 minutes) shown on the calendar - that would be a nice feature for Google to add.