Is it normal to take days to compile PostgreSQL from source code?
How long it takes to compile will depend on a big number of factors, mostly the hardware (or resources of your virtual machine) on which you're compiling, plus your OS and environment settings, plus what else is your computer doing while you compile.

To have a reference of times, you can check PostgreSQL BuildFarm.

For instance, for a certain system called gull, which runs Debian 8.3, the gull PostgreSQL BuildFarm for date 2017-07-06 shows a Total run time
01:05:48. 

Your system might actually be an order of magnitude slower and take 12 hours. It seems a bit "overkill", but possible. Although I don't know the specifics of compiling PostgreSQL for Debian (specially, how parallel it can be made), I'm sure that more memory and more (and faster) cores will decrease compilation time. It is also very likely that, at this phase of development, some settings to ease debugging might be different from releases on production, and (maybe) take longer to compile.

As for packages already compiled, check 


Search PostgreSQL at debian.org
Package postgresql-10 (10~beta1-1)