Best advice is to change it at the maintenance interval listed in your owners manual. It's alway's better to change it early to prevent problems than to wait until it doesn't work and have to troubleshoot the problem and fix it.
I have an JD HPX Diesel. I change the oil(2qts), oil and fuel filter every 50 hrs. Also change the air filter (inner and outer) every 100 hrs. The fuel filter is too cheap and important not to change it with every oil change. I think Kubota's fuel filter is about the same as what JD uses.
The Kubota RTV900 service manual says to check condition of fuel lines and filter every year but if no problems are found that you'll only need to change the fuel filter every 500 hours.
The generic service schedule on the Kubota website - this covers all their equipment - says to change the fuel filter every 300 hours.