Well I would give this to the team and ask them to give estimates to the given test cases and then I will evaluate whether the estimates provided by them are correct or not.

If a tester gives, more estimates from other tester try to find why his estimates are more than other may be he is doing the detail downstream testing.

Usually if its a function test cast it will not take more than 5 min to each test case which includes +VE, -VE, border, special character testing.

If it is a downstream testing, it will take 20 min to execute each TC.