My answer, to help people on getting a better approx time for the process.
I have just converted a 1 TB external hard drive from fat32 to ntfs, having about 200 GB of data, and it took about 50 minutes.
however, instead of directly using a command in windows (seven), i used this tool: partition wizard. Anyway, i guess the only difference is that those partition utilities (there are several similar utils, like easeus ) use the built in windows commands, but they do it for you, so you only have to give a few clicks in a nice UI
I was afraid of losing data, since the drive is actually for backups, but it went pretty well, and i expected it to last MUCH longer (i imagined like 3 hours or something). That is precisely why i am giving this answer, to let people better estimate the time it will take for their cases.
For your case, with 500 GB of used space, it may take a couple of hours.
Anyway, as other answers say, there is always a risk, and it is a good idea of having a backup elsewhere, or just take the risk, but don't complain if something gets wrong =)
Oh, and what not everybody mentions, is that you NEED to have some free space in the drive/partition to make the conversion, if you have an almost full disk, it wont let you continue. In my case, having 200 GB used of 1000 GB, it asked for about 1.5 GB of free space, so i had no problem, but other cases may differ.