Of course, the answer is “that depends”.
1. Do you already have downloads of files to be installed. This alone can take hours if you are downloading from MSDN subscription library (for example: Win2008, SQL 2008, Visual Studio, and BizTalk, and sometimes additiona prereqs an Ser). I would allow about 4-8 hours just for these downloads, but of course, you can be working on other things while they are downloading. If you are fortunate to have all these files ready to go, you will save time.
2. What are you starting with ?
a) Has someone already installed Windows for you?
b) Is IIS installed or enabled on Windows ?
c) Are you connecting to an existing SQL server, or are you going to install SQL also.
d) Is the appropriate version of Visual Studio already installed?
3. BizTalk install itself takes only about 10 minutes, but after that, you have to run the Configure utility. This usually takes 15-45 minutes, when things work well. When you have errors, and have to re-run and investigate the logs, it obviously takes longer.
Here’s a big picture example. This week, I started with “nothing”, and it took me about 3 days (part time) to install two virtual machines. One was BT2009 with SQL 2009 (not R2), and another with BT2010, VS2010, and SQL2008R2.
So when your boss asks you, “How long will it take to install BizTalk”, you can reply with some of the questions above, and hopefully give him a more intelligent answer.