Thanks for your posting. Estimates on the test suite level are calculated based on the estimates of their test cases, so there’s currently no way to specify the estimate on the test suite level itself. But I agree that this would be useful to have, especially if your test cases are very short to execute, and I’ve added this as a feature request.
That said, if you are mainly interested in the estimates because of TestRail’s forecast and burndown features (on the Progress pages for test runs, plan and milestones), you wouldn’t have to enter (accurate) estimates for all your test cases. TestRail can calculate averages for test cases (based on the estimates of other test cases in the same test suite) and also uses the ‘elapsed’ values of test results. Please see the online help of TestRail for details (Help link in TestRail in the upper right corner > TestRail Help | Usage tips & tricks | Scheduling & Forecasting).