A more precise answer would be, download the Getright software you will find on the same Steel Panthers download page that you will find Steel Panthers.
The good news is that, Getright will download safely your file regardless of how many interruptions you experience. So if you can only afford an hour an evening no problem.
The bad news is that you wont be happy with just the 5.1 after you have played the game a while. You will be tempted to get perhaps the upgrades to 6.1
Steel Panthers is a file that self-extracts for you, but its a LARGE file. The last time I downloaded Steel Panthers it had grown to several hundred mags (think something between 650 and 700 megs).
Do NOT alter its path in any way when downloading. Yes some of us like to give our files custom locations, resist this idea.
Steel Panthers downloads best when you agree to put everything EXACTLT where it wants to on your hard drive. Just click Yes or Ok to everything and you wont suffer any headaches.
This isnt a tech level piece of advice, merely the observation of a person that has installed Steel Panthers any version on everything from Win95/98/XP OS using computers (I get no hassles when I dont fuss with the install).
This link should get you all the Steel Panthers you can handle. I also suggest getting the manual file as well.
http://www.matrixgames.com/games/WorldAtWar/downloads.asp
As for actual time to download. Depends on your connection quality (not all locations are created equal). Country you are in, (some countries dont have great service period), your actual connection type (I have a DSL connection, almost equal to cable), time of day, (yep this counts), and it might be important to remember that its evening to you but morning somewhere else.
On a good day I can download awesome amounts of data. Steel Panthers is possible for me in 90 minutes. But with my original ordinary phone connection I was without a phone for 3 solid days once (which is why you might like using Getright when you go to bed).
My best day so far...I downloaded 5 Gigs of video data during ordinary daylight hours. Anything is possible eh.
Hope this has been useful. You will shortly notice that the Matrix site has several other programs you are going to want. You will be needing that Getright download managing software I think.