How long is a 'ride' anyway?

As I try to figure out which oil to use and I search through the many posts here, I often here things like 'change your tranny oil after every second ride', others do it after every ride.

My question is: how long is a ride anyway? Are these people getting up at 6AM, hopping on their bikes and cruising on trails till sundown on a long summer day? Or are they riding a couple of laps around the motocross track and counting that as a 'ride'?

I mean, talk about a vague description. Like, those that say you should change your oil as often as possible. Well, that would mean that as soon as do an oil change and start up the bike, I should turn it off and do a new oil change. Again, very vague directions.

So back to my question: how long is this ride you talk about anyway? One hour? 12 hours? What type of conditions: slowly cruising the trails in low third or full blast at the mx track?

If instead of using the term 'ride' people would be a little more descriptive such as in the number of actual hours of riding, or counting that as a tank full, it would be far less confusing.

For instance, when I fill my gas tank and do laps at the motocross track (not going hard) it usually lasts me 2.5 hours or so of casual cruising the bumps with the eventual drink breaks.
Using the gas tank as a scale, how many gas tanks can I go through before having to change the oil? I have just added Castrol Grand Prix 4 stoke 10-40 oil to my rm 250. Not sure when to change it next and would like to have a good indication as to when that should take place.
Just please don't tell after every 'ride'.

End of rant.

Thanks all for your responses, this gives me a better idea.
For the past couple of seasons I have been running Yamalube 4 stroke oil and would leave it in the bike for about 20 hours, which I see now to be way too long. In fact the oil is often dark grey and this can't be good. If this Castrol Grand Prix works well (should find out in a day or two) then I'll be replacing the oil a lot more often, since it costs less than half the Yamalube.

I'll go with va_yzrider's numbers and will aim for a change after my third time out, which should total about 10 hours to 12 hours of doing laps at the mx track (I don't ride hard, so this could probably be enlongated a bit). But this gives me a starting point.
Thanks again. Will let you know how I find the Castrol Grand Prix oil (which btw can be found at Walmart, at least here in Canada)