If you don't have any service records then change the fluids right away , or before a long trip.
Then depending on , miles of use and fluid condition, 3years 24,000 miles on the trans and diff., A/Freeze, coolant as per manufacturer, Engine oil, I'm assuming gas here, 3,>5,000 miles on trips or when using frequently, or before any lengthy period of storeage, to get contaminants out of the pan, before they have a chance to sludge up. Oil types , well your going to hear from a lot of people on that. My 2c worth, follow the chassis makers recommendations.
BTW; Welcome to iRV2.