The time it takes really depends on the system you're installing it to, the stuff you choose to install, the size of your databases, a ton of stuff that I couldn't possibly quantify for you. Best thing to do is test it out. As for precautions, make darned sure you have backups in place and that you know how to run a restore. Also make sure you have the original media so you can reinstall if needed. But that's the same thing you should have before you do patching or anything else like that.