I've been waiting for a little over an hour for it to install. I am using a PowerMac G5 dual-core with leopard.
It all depends on your internet speed, such as a slow 3mbps (megabits, not megabytes per second) or a fast 40mbps. You can calculate the time it should take by dividing your connection speed (in megabits) by 8 (8 bits in a byte), then dividing your download size (in megabytes) by that number. I use an iPhone app called Ookla Speedtest, it's free, but ad-supported and seems to work well.