badblocks taking really long time. or is it frozen?

You can watch per-process I/O activity with iotop. Install:

sudo apt-get install iotop

Then launch:

sudo iotop

Watch it for 30 sec. Does it show badblocks doing any I/O? If yes, then it is working, if not, it is likely frozen.

Next time you may launch badblocks with the -s parameter:

sudo badblocks -nsv /dev/sda6

this will show progress.

You can see where badblocks is currently positioned in the drive by looking at its fdinfo in proc, i.e:

root@ubuntu:/home/ubuntu# ps ax | grep badblock
 3772 tty1     D+    12:11 badblocks -v /dev/sdb
 5478 tty2     S+     0:00 grep --color=auto badblock

root@ubuntu:/home/ubuntu# ls -l /proc/3772/fd
total 0
lrwx------ 1 root root 64 Mar  3 00:41 0 -> /dev/tty1
lrwx------ 1 root root 64 Mar  3 00:41 1 -> /dev/tty1
lrwx------ 1 root root 64 Mar  3 00:41 2 -> /dev/tty1
lr-x------ 1 root root 64 Mar  3 00:41 3 -> /dev/sdb

root@ubuntu:/home/ubuntu# cat /proc/3772/fdinfo/3
pos:    1707980816384
flags:  0140000
mnt_id: 24

root@ubuntu:/home/ubuntu# lsblk -bd /dev/sdb
NAME   MAJ:MIN RM          SIZE RO TYPE MOUNTPOINT
sdb      8:16   0 2000398934016  0 disk 

So, this badblocks run is about 85% done.

@falconer's answer (use iotop) is a nice way to watch its current rate, but fdinfo shows you how much progress the program has made.