I've read it on these forums over and over about riders changing their oil on every ride, every other ride, or some other forum of willy-nilly interval.
I'm all for making sure I'm running good, clean oil, but I believe that too many are changing their oil unnecessarily often, costing them both time and money that could be used for gas to the track or trail or other necessary maintenance.
If you're out riding from 9am to 4pm, chances are your motor wasn't running the whole time. So, you didn't ride for 7 hours, more like 4-5 running hours and that's what counts when it comes to changing your oil.
That said, either pay attention to your hour meter or install an hour meter and follow the interval in your manual. Or, if you're especially anal, reduce your manufacturer's recommendation by 10%. So, for example, if your manual says every 15 hours, go with 13.5. However, if you're racing, every single advantage matters, so changing oil after every race MIGHT make sense. If you really want to know, have the oil analyzed. This is the ONLY way to truly know the condition of the oil.
I guess my real point here is to keep tracking of running hours, not the time between when you left and came back. Trust me, your motor wasn't running nearly as much as you are estimating and considering the increasing cost of just riding, you'll appreciate the money saved and you can use the time saved for writing letters to your elected officials in the effort to preserve our riding areas.