No real way to tell. Steam has a cap on how fast it downloads games but location and other network issues could cause some pretty major variations in download times.
I have 200mbps download and if I'm lucky I can reach 10mbps. The Steam servers are loaded a lot of time since it's a huge platform, so it can't really afford to give everyone great download speeds. I'd expect 2-3mpbs average and 5 if you're downloading at a really good time.