It is not 10-15 minutes from full to nothing. They way it works is that you get 2500 points in gathering. Every time you gather something you go down 10 points if you get something from it. Every hour you get 100 points back. This means you get to gather 250 itmes per 25 hours before you cannot get anything.

Once you get down to 1000 of your 2500 points it gets a bit harder to get items but you still can.

As a high ranked gather guy myself I recomend you make it easy on yourself and gather once a day at the same general time every day that way you stay at near 2500 points every day (this will help avoid the 10-15 minutes issue)

Another think that will help you to feel a bit better is if you level Mining and Botany at the same time. That way you can just macro in your equip changes and have 2500 points in each every 25 hours.

Gathering is fun and not all that hard to level up if you just make it a once a day thing for a few hours. I hope this information helps and keep at it gathering rocks!