Um, there's actually a bit in the status conditions section of the Database. Ha ha. Am I the only one who looked through all the features to see what they were all about?
Open your database.
Click the "States" tab.
Create a new state titled "Double Gold".
Go over to "Features".
Right click and open it up to add a new feature.
Click the "Other" tab.
Click the "Party Ability" dot.
Scroll down to "Gold Double" in the list, should be second one from the bottom.
Now, go into common events. Set an event so that when a switch or variable is triggered, it inflicts this new status upon your party.
After you're done with that, you need to set up some variables to create a timer.
http://blog.rpgmakerweb.com/tutorials/tracking-time-in-common-events/ This is the web address I was given when I couldn't figure out how to make a timer last more than 99 minutes and needed it to last 5 hours instead.
After you have your timer set up and running (which only starts running if your variable or switch is the conditional and is activated), create a conditional branch that basically says "once timer reaches x number, reset variable/switch and remove state".