An Instructional Tease: Advanced Anal: Household toys pt.2: Ice!
If you went to the bathroom, then sit on the toilet, but try not to let them out. See if you can let them melt first, just for fun. You can relieve some pressure if you need, and let it all go when you can't hold it anymore.
In terms of safety, it doesn't matter how long you hold it for or anything. In fact, if you just stuck one or two in, your body might just absorb the water through your rectum before you have to push it out. Ice is one of the safest things to stick up your ass, so don't worry about it!
If you're still on your towel, with five cubes in your ass, congratulations. Stand up now, over your towel. Is there cold water running down your leg, or are you keeping it all in? While standing, with your legs apart, finger your ass one last time while you stroke yourself.