The steps to use intervalometer -
Shooting Menu -> Interval Timer Shooting -> Start time(Basically select Now) -> Interval -> Select intervals X no. of shots -> Start(Basically select On and press ok button if you want to start now)
Interval = Shutter Speed + Interval Between Shot
Select intervals X no. of Shots = Number Of Interval x Number of shot
To configure the setting, you first need to know your shutter speed, say your shutter speed is 10 seconds, and you want 5 seconds interval between each shot, you need to set 15 seconds in Interval.
For shooting total 50 frames, what you need to do is set Select intervals to 50 and no. of shots to 1 so it becomes 50 x 1 = Total 50 frames.
Basically this means that it will run 50 times of 15 seconds interval so you get the idea of total time too.
Unless my shutter speed needs to go over 30 seconds since that's the max for D7000, otherwise there's no point for me to use external remote device for the job. Cheers ;)