Hello...hey - tell us your name when you post or better yet go ahead when you can and complete at least a little of your profile.
In response to your question...I concur with Linda. To put this into perspective think about what any other professional does. An attorney usually charges in 6 minute increments based on his/her hourly. A doctor does not care if he spends 1 minute with you or 20...it's still the same set fee.
You can look at it this way as well... If you have 10 clients and they all tap you consistently to do little 5 minute things here and there and do not expect to get charged then that is almost an hour of your time.
It is very important that you do not try to convince your clients of anything - and the information I provide you is for your mindset alone and should not be used in conversations with them. Set your policies, make sure expectations are clear in your own mind, and if the client balks at you billing for 15 minutes for short jobs then that is definitely not a client that you want because they are going to nickel and dime you to death. If you are confident and clear in your own mind that you are doing the right thing - the right people won't even think twice about it.
Hope this helps.