My computer is already quite old and since it only had 320 GB of space which should be divided for Windows and Linux, i didn't allocate much for Linux during partitioning the hard drive. I only gave 55 GB for Linux, 80 GB for Windows (C), 100 GB for Master (D), and 70 GB for Data (E). Both D and E were shared with Linux since reading/writing NTFS partition is no longer a problem with Linux. At that time, i didn't think of using virtualizations and building so many packages like nowadays.
Things changed when i joined Chess Griffin to work on MSB project, providing MATE packages for Slackware 14.0 and 14.1. The packages must be built on a clean environment, so i had to use virtualization technology to build them. Since it should be built on two arch (x86 and x86_64), i had to make 2 VMs for that. I'm still able to manage things at that point as i only need like 17 GB for 2 Slackware VMs built using a full clean installation of Slackware 14.0 (now 14.1), but i had to clean up some packages which i rarely used.
On November 2013, i joined the SlackBuilds project as the team member after Robby Workman invited me and at that time, i was thinking of buying a new disk for saving the sources and also the files created during build process. Testing a package sometimes really takes a lot of space since sometimes it requires so many dependencies and it takes a lot of space and time to build a single package.
Today, i made up my mind to buy a new hard drive for my desktop at home and i bought 2 TB of hard drive which should be more than enough. I already formatted the disk and prepare all the partitions. Suppose my old hard drive is dead, i can just install the OS onto this new partition and all of my data has been transfered there as backup. I have allocated 500 GB for build partition and another 500 GB for VM partition, which should be more than enough for MSB and SBo project (and probably other future project).
I'm still using my old disk together with the new one and it's already synchronized in most of the data contained, but i still have plenty of room in the new disk.