I've just installed Ubuntu Mate on my Raspberry Pi 2. I've selected Performance governor on cpufreq so it will run on 900 Mhz all the time. But I'm wondering is it safe for long periods like 8 - 10 hours? Could it overheat?
900 Mhz is the default clock speed for the Pi 2. As this is the designed clock speed no damage will be done, nor should you have any overheating problems (assuming you are not operating it in a high temp area (outside in the desert, next to an industrial oven etc.).
Personally I have run my various Pi's at up to 1000mHz without problem 24x7 for months at a time and never seen 50 degrees Celsius.